diff --git a/src/settings/Settings.ts b/src/settings/Settings.ts
index ca5e2f1d04..ea58201322 100644
--- a/src/settings/Settings.ts
+++ b/src/settings/Settings.ts
@@ -38,6 +38,7 @@ import { UIFeature } from "./UIFeature";
 import { OrderedMultiController } from "./controllers/OrderedMultiController";
 import {Layout} from "./Layout";
 import ReducedMotionController from './controllers/ReducedMotionController';
+import IncompatibleController from "./controllers/IncompatibleController";
 
 // These are just a bunch of helper arrays to avoid copy/pasting a bunch of times
 const LEVELS_ROOM_SETTINGS = [
@@ -188,6 +189,8 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         displayName: _td("Show message previews for reactions in DMs"),
         supportedLevels: LEVELS_FEATURE,
         default: false,
+        // this option is a subset of `feature_roomlist_preview_reactions_all` so disable it when that one is enabled
+        controller: new IncompatibleController("feature_roomlist_preview_reactions_all"),
     },
     "feature_roomlist_preview_reactions_all": {
         isFeature: true,
diff --git a/src/settings/controllers/IncompatibleController.ts b/src/settings/controllers/IncompatibleController.ts
new file mode 100644
index 0000000000..c48ce0a60b
--- /dev/null
+++ b/src/settings/controllers/IncompatibleController.ts
@@ -0,0 +1,46 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import SettingController from "./SettingController";
+import { SettingLevel } from "../SettingLevel";
+import SettingsStore from "../SettingsStore";
+
+/**
+ * Enforces that a boolean setting cannot be enabled if the incompatible setting
+ * is also enabled, to prevent cascading undefined behaviour between conflicting
+ * labs flags.
+ */
+export default class IncompatibleController extends SettingController {
+    public constructor(private settingName: string, private forcedValue = false) {
+        super();
+    }
+
+    public getValueOverride(
+        level: SettingLevel,
+        roomId: string,
+        calculatedValue: any,
+        calculatedAtLevel: SettingLevel,
+    ): any {
+        if (this.incompatibleSettingEnabled) {
+            return this.forcedValue;
+        }
+        return null; // no override
+    }
+
+    public get incompatibleSettingEnabled(): boolean {
+        return SettingsStore.getValue(this.settingName);
+    }
+}