Randomly, without touching anything sometimes (like, say my wife is reading a page with her hands away from the keyboard and touchpad) her Tab Group will randomly switch. If she is in an App Tab, she will stay in that App Tab and nothing else happens (which is the only nice thing), but if she is in a non-App Tab when it happens, then the tab she is in switches over to the alternate Tab Group with her, and a random tab from the alternate Tab Group gets sent to the original Tab Group.

Kinda makes liking Tab Groups hard for the poor girl :(