diff --git a/doc/classes/ProjectSettings.xml b/doc/classes/ProjectSettings.xml
index ffb61ab20d3..57387aa65ad 100644
--- a/doc/classes/ProjectSettings.xml
+++ b/doc/classes/ProjectSettings.xml
@@ -2226,7 +2226,8 @@
 			[b]Note:[/b] Depth prepass is only supported when using the Forward+ or Compatibility rendering method. When using the Mobile rendering method, there is no depth prepass performed.
 		</member>
 		<member name="rendering/driver/threads/thread_model" type="int" setter="" getter="" default="1">
-			Thread model for rendering. Rendering on a thread can vastly improve performance, but synchronizing to the main thread can cause a bit more jitter.
+			The thread model to use for rendering. Rendering on a thread may improve performance, but synchronizing to the main thread can cause a bit more jitter.
+			[b]Note:[/b] The [b]Multi-Threaded[/b] option is experimental, and has several known bugs which can lead to crashing, especially when using particles or resizing the window. Not recommended for use in production at this stage.
 		</member>
 		<member name="rendering/environment/defaults/default_clear_color" type="Color" setter="" getter="" default="Color(0.3, 0.3, 0.3, 1)">
 			Default background clear color. Overridable per [Viewport] using its [Environment]. See [member Environment.background_mode] and [member Environment.background_color] in particular. To change this default color programmatically, use [method RenderingServer.set_default_clear_color].
diff --git a/main/main.cpp b/main/main.cpp
index b759944ff03..f7de02fb733 100644
--- a/main/main.cpp
+++ b/main/main.cpp
@@ -1150,6 +1150,9 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 					rtm = OS::RENDER_THREAD_UNSAFE;
 				} else if (I->next()->get() == "separate") {
 					rtm = OS::RENDER_SEPARATE_THREAD;
+				} else {
+					OS::get_singleton()->print("Unknown render thread mode, aborting.\nValid options are 'unsafe', 'safe' and 'separate'.\n");
+					goto error;
 				}
 
 				N = I->next()->next();
@@ -1875,7 +1878,8 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 	}
 
 	if (rtm >= 0 && rtm < 3) {
-		if (editor) {
+		if (editor || project_manager) {
+			// Editor and project manager cannot run with rendering in a separate thread (they will crash on startup).
 			rtm = OS::RENDER_THREAD_SAFE;
 		}
 		OS::get_singleton()->_render_thread_mode = OS::RenderThreadMode(rtm);
@@ -2209,6 +2213,10 @@ Error Main::setup2() {
 		}
 	}
 
+	if (OS::get_singleton()->_render_thread_mode == OS::RENDER_SEPARATE_THREAD) {
+		WARN_PRINT("The Multi-Threaded rendering thread model is experimental, and has known issues which can lead to project crashes. Use the Single-Safe option in the project settings instead.");
+	}
+
 	/* Initialize Pen Tablet Driver */
 
 	{