When you quickly dock a tab to the main window, you can sometimes end up with 2 identical tabs. This causes some weird behavior, such as every action being done in one window being replicated in the other. If the undocked window is closed, it can cause the tabs in the main window to become dysfunctional.
Note: While this does happen in 4.11.2, the result is a bit different in that the duplicate window shows up undocked and is extremely small.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34895 in the post.