When opening a material in the material editor, there is a chance that the material editor will open and become a tab on the main editor window even if a material was already opened and detached from the main editor window.
1. Open Editor (any project)
2. Open a material
3. Detach material editor window from main editor window
4. Close the material
5. Open another material
6. Open a second material at the same time
7. Repeat steps 4-6 several times
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-13329 in the post.