I'm on Mac. A bit of a fringe case, but:
- drag your mouse to select a bit of text. don't let go of the mouse cursor (so you're still dragging/selecting)
- press the undo-shortcut
Weird things start happening (whole blocks of text vanish, redo/undo seem to behave kind of erratically).
In other programs, the undo shotcut does nothing in the same scenario. This seems to be preferrable, because I like my undo/redo to behave very predictable.