If you attempt to edit a prefab with a UI Scrollbar component attached while in playmode, you'll get the following warning:

Prefab Mode in Play Mode was blocked by the script 'Scrollbar' to prevent the script accidentally affecting Play Mode. See the documentation for [ExecuteInEditMode] and [ExecuteAlways] for info on how to make scripts compatible with Prefab Mode during Play Mode.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Looks like this first appeared in 2019.1.0a5

1. Install Unity
2. Open the attached project
3. Enter Play Mode
3. Navigate to Assets > Canvas.prefab
4. Attempt to open Canvas.prefab in isolation mode

Expected Result:
The prefab is opened in isolation mode for editing.

Actual Result:
Isolation mode is blocked from opening due to the Scrollbar component being present on the prefab.

Tested on Windows.

Occurs on 2019.2.0a1, 2019.1.0a14, 2019.1.0a5

Does not occur on 2018.3.0f2, 2019.1.0a4