Ensure gameplay can't start when an UnknownMod is present

osu.Game.Tests/Visual/Gameplay/UnknownModTestScene.cs

@@ -0,0 +1,26 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using NUnit.Framework;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Osu;
+
+namespace osu.Game.Tests.Visual.Gameplay
+{
+    public class TestSceneUnknownMod : ModTestScene
+    {
+        protected override Ruleset CreatePlayerRuleset() => new OsuRuleset();
+
+        [Test]
+        public void TestUnknownModDoesntEnterGameplay()
+        {
+            CreateModTest(new ModTestData
+            {
+                Beatmap = CreateWorkingBeatmap(new OsuRuleset().RulesetInfo).Beatmap,
+                Mod = new UnknownMod("WNG"),
+                PassCondition = () => Player.IsLoaded && !Player.LoadedBeatmapSuccessfully
+            });
+        }
+    }
+}

osu.Game/Screens/Play/Player.cs

@@ -185,6 +185,12 @@ namespace osu.Game.Screens.Play
         {
             var gameplayMods = Mods.Value.Select(m => m.DeepClone()).ToArray();
 
+            if (gameplayMods.Any(m => m is UnknownMod))
+            {
+                Logger.Log("Gameplay was started with an unknown mod applied.", level: LogLevel.Important);
+                return;
+            }
+
             if (Beatmap.Value is DummyWorkingBeatmap)
                 return;