Version: M54beta (but has been happening for a while now)
OS: macOS 10.11.6

- open a window (A) with some tabs
- open a new window (B)
- load youtube.com in window B
- click one of the videos to start playing it
- fullscreen the video using the widget in the player (not the traffic lights). This creates a new OS space with the window.
- switch spaces back to the previous space and window A.
- wait a few seconds

The OS will switch the active space back to the fullscreen YouTube video without doing anything. I can repeat this pretty easily.