diff --git a/lib/ot/client.js b/lib/ot/client.js
index 7ee19fdc..c4384e88 100755
--- a/lib/ot/client.js
+++ b/lib/ot/client.js
@@ -219,19 +219,26 @@ ot.Client = (function (global) {
     this.acknowlaged = acknowlaged;
     this.client = client;
     this.revision = revision;
+    this.pendingOps = [];
   }
   Client.Stale = Stale;
 
   Stale.prototype.applyClient = function (client, operation) {
-    return new StaleWithBuffer(this.acknowlaged, operation, client, this.revision);
+    var swb = new StaleWithBuffer(this.acknowlaged, operation, client, this.revision);
+    swb.pendingOps = this.pendingOps;
+    return swb;
   };
 
   Stale.prototype.applyServer = function (client, revision, operation) {
-    throw new Error("Ignored server-side change.");
+    // Buffer incoming operations while waiting for the missing operations
+    // response. These will be applied sequentially after the missing ops.
+    this.pendingOps.push({ revision: revision, operation: operation });
+    return this;
   };
 
   Stale.prototype.applyOperations = function (client, head, operations) {
     var transform = this.acknowlaged.constructor.transform;
+    // Apply the requested missing operations (concurrent with acknowlaged)
     for (var i = 0; i < operations.length; i++) {
       var op = ot.TextOperation.fromJSON(operations[i]);
       var pair = transform(this.acknowlaged, op);
@@ -239,6 +246,14 @@ ot.Client = (function (global) {
       this.acknowlaged = pair[0];
     }
     client.revision = this.revision;
+    // Apply any operations that arrived while we were waiting for the
+    // missing operations. These are sequential (not concurrent), so they
+    // can be applied directly like Synchronized.applyServer would.
+    for (var j = 0; j < this.pendingOps.length; j++) {
+      var pending = this.pendingOps[j];
+      client.revision = pending.revision;
+      client.applyOperation(pending.operation);
+    }
     return synchronized_;
   };
 
@@ -261,6 +276,7 @@ ot.Client = (function (global) {
     this.buffer = buffer;
     this.client = client;
     this.revision = revision;
+    this.pendingOps = [];
   }
   Client.StaleWithBuffer = StaleWithBuffer;
 
@@ -270,11 +286,15 @@ ot.Client = (function (global) {
   };
 
   StaleWithBuffer.prototype.applyServer = function (client, revision, operation) {
-    throw new Error("Ignored server-side change.");
+    // Buffer incoming operations while waiting for the missing operations
+    // response. These will be applied sequentially after the missing ops.
+    this.pendingOps.push({ revision: revision, operation: operation });
+    return this;
   };
 
   StaleWithBuffer.prototype.applyOperations = function (client, head, operations) {
     var transform = this.acknowlaged.constructor.transform;
+    // Apply the requested missing operations (concurrent with acknowlaged and buffer)
     for (var i = 0; i < operations.length; i++) {
       var op = ot.TextOperation.fromJSON(operations[i]);
       var pair1 = transform(this.acknowlaged, op);
@@ -284,6 +304,16 @@ ot.Client = (function (global) {
       this.buffer = pair2[0];
     }
     client.revision = this.revision;
+    // Apply any operations that arrived while we were waiting for the
+    // missing operations. These are sequential (not concurrent), so we
+    // transform the buffer against them and apply them to the document.
+    for (var j = 0; j < this.pendingOps.length; j++) {
+      var pending = this.pendingOps[j];
+      var pair3 = transform(this.buffer, pending.operation);
+      client.applyOperation(pair3[1]);
+      this.buffer = pair3[0];
+      client.revision = pending.revision;
+    }
     client.sendOperation(client.revision, this.buffer);
     return new AwaitingConfirm(this.buffer);
   };
diff --git a/lib/ot/editor-socketio-server.js b/lib/ot/editor-socketio-server.js
index 5014ac8d..cef29db7 100755
--- a/lib/ot/editor-socketio-server.js
+++ b/lib/ot/editor-socketio-server.js
@@ -51,14 +51,23 @@ EditorSocketIOServer.prototype.addClient = function (socket) {
                 self.onOperation(socket, revision, operation, selection);
                 if (typeof self.operationCallback === 'function')
                     self.operationCallback(socket, operation);
+                socket._opRetries = 0;
             } catch (err) {
+                logger.error("Operation error from client " + socket.id + ": " + err.message);
+                if (!socket._opRetries) socket._opRetries = 0;
+                socket._opRetries++;
                 setTimeout(function() {
                     var docOut = {
                         str: self.document,
                         revision: self.operations.length,
-                        clients: self.users,
-                        force: true
+                        clients: self.users
                     };
+                    // Only force-reset after 3 consecutive failures as a last resort.
+                    // Without force:true, the client will use its Stale recovery to resync.
+                    if (socket._opRetries >= 3) {
+                        docOut.force = true;
+                        socket._opRetries = 0;
+                    }
                     socket.emit('doc', docOut);
                 }, 100);
             }
diff --git a/public/docs/release-notes.md b/public/docs/release-notes.md
index f7f4ddc6..797d5cb8 100644
--- a/public/docs/release-notes.md
+++ b/public/docs/release-notes.md
@@ -1,5 +1,11 @@
 # Release Notes
 
+## <i class="fa fa-tag"></i> 1.x.x <i class="fa fa-calendar-o"></i> UNRELEASED
+
+### Bugfixes
+- Fix data loss when 5+ users edit a document concurrently, caused by the OT client discarding operations during revision gap recovery ([#6342](https://github.com/hedgedoc/hedgedoc/issues/6342))
+- Add defensive null checks to `hex2rgb` to prevent crashes from non-hex color values
+
 ## <i class="fa fa-tag"></i> 1.10.7 <i class="fa fa-calendar-o"></i> 2026-02-24
 
 ### Bugfixes
diff --git a/public/vendor/ot/client.js b/public/vendor/ot/client.js
index 7ee19fdc..c4384e88 100644
--- a/public/vendor/ot/client.js
+++ b/public/vendor/ot/client.js
@@ -219,19 +219,26 @@ ot.Client = (function (global) {
     this.acknowlaged = acknowlaged;
     this.client = client;
     this.revision = revision;
+    this.pendingOps = [];
   }
   Client.Stale = Stale;
 
   Stale.prototype.applyClient = function (client, operation) {
-    return new StaleWithBuffer(this.acknowlaged, operation, client, this.revision);
+    var swb = new StaleWithBuffer(this.acknowlaged, operation, client, this.revision);
+    swb.pendingOps = this.pendingOps;
+    return swb;
   };
 
   Stale.prototype.applyServer = function (client, revision, operation) {
-    throw new Error("Ignored server-side change.");
+    // Buffer incoming operations while waiting for the missing operations
+    // response. These will be applied sequentially after the missing ops.
+    this.pendingOps.push({ revision: revision, operation: operation });
+    return this;
   };
 
   Stale.prototype.applyOperations = function (client, head, operations) {
     var transform = this.acknowlaged.constructor.transform;
+    // Apply the requested missing operations (concurrent with acknowlaged)
     for (var i = 0; i < operations.length; i++) {
       var op = ot.TextOperation.fromJSON(operations[i]);
       var pair = transform(this.acknowlaged, op);
@@ -239,6 +246,14 @@ ot.Client = (function (global) {
       this.acknowlaged = pair[0];
     }
     client.revision = this.revision;
+    // Apply any operations that arrived while we were waiting for the
+    // missing operations. These are sequential (not concurrent), so they
+    // can be applied directly like Synchronized.applyServer would.
+    for (var j = 0; j < this.pendingOps.length; j++) {
+      var pending = this.pendingOps[j];
+      client.revision = pending.revision;
+      client.applyOperation(pending.operation);
+    }
     return synchronized_;
   };
 
@@ -261,6 +276,7 @@ ot.Client = (function (global) {
     this.buffer = buffer;
     this.client = client;
     this.revision = revision;
+    this.pendingOps = [];
   }
   Client.StaleWithBuffer = StaleWithBuffer;
 
@@ -270,11 +286,15 @@ ot.Client = (function (global) {
   };
 
   StaleWithBuffer.prototype.applyServer = function (client, revision, operation) {
-    throw new Error("Ignored server-side change.");
+    // Buffer incoming operations while waiting for the missing operations
+    // response. These will be applied sequentially after the missing ops.
+    this.pendingOps.push({ revision: revision, operation: operation });
+    return this;
   };
 
   StaleWithBuffer.prototype.applyOperations = function (client, head, operations) {
     var transform = this.acknowlaged.constructor.transform;
+    // Apply the requested missing operations (concurrent with acknowlaged and buffer)
     for (var i = 0; i < operations.length; i++) {
       var op = ot.TextOperation.fromJSON(operations[i]);
       var pair1 = transform(this.acknowlaged, op);
@@ -284,6 +304,16 @@ ot.Client = (function (global) {
       this.buffer = pair2[0];
     }
     client.revision = this.revision;
+    // Apply any operations that arrived while we were waiting for the
+    // missing operations. These are sequential (not concurrent), so we
+    // transform the buffer against them and apply them to the document.
+    for (var j = 0; j < this.pendingOps.length; j++) {
+      var pending = this.pendingOps[j];
+      var pair3 = transform(this.buffer, pending.operation);
+      client.applyOperation(pair3[1]);
+      this.buffer = pair3[0];
+      client.revision = pending.revision;
+    }
     client.sendOperation(client.revision, this.buffer);
     return new AwaitingConfirm(this.buffer);
   };
diff --git a/public/vendor/ot/hex2rgb.js b/public/vendor/ot/hex2rgb.js
index 176a0f2f..7e1de864 100644
--- a/public/vendor/ot/hex2rgb.js
+++ b/public/vendor/ot/hex2rgb.js
@@ -1,12 +1,17 @@
 function hex2rgb (hex) {
+  if (!hex || typeof hex !== 'string') return { red: 0, green: 0, blue: 0 }
   if (hex[0] == '#') hex = hex.substr(1)
   if (hex.length == 3) {
     var temp = hex
     hex = ''
-    temp = /^([a-f0-9])([a-f0-9])([a-f0-9])$/i.exec(temp).slice(1)
+    var match = /^([a-f0-9])([a-f0-9])([a-f0-9])$/i.exec(temp)
+    if (!match) return { red: 0, green: 0, blue: 0 }
+    temp = match.slice(1)
     for (var i = 0; i < 3; i++) hex += temp[i] + temp[i]
   }
-  var triplets = /^([a-f0-9]{2})([a-f0-9]{2})([a-f0-9]{2})$/i.exec(hex).slice(1)
+  var result = /^([a-f0-9]{2})([a-f0-9]{2})([a-f0-9]{2})$/i.exec(hex)
+  if (!result) return { red: 0, green: 0, blue: 0 }
+  var triplets = result.slice(1)
   return {
     red: parseInt(triplets[0], 16),
     green: parseInt(triplets[1], 16),