When you notice that some small aspect of reality has suddenly changed from what you thought it was, like in the Star Trek: TNG episode Parallels when Worf silently switches between alternate realities.
Sean: Whoa, since when have Disaronno bottles been rhombuses? I thought they were rectangular.
Liam: Dude, they've always been rhombuses.
Sean: I think I'm worfing.
Worf: I... I thought the cake was chocolate.
Troi: Don't I wish!
Worf: Counselor, the painting that Commander Data gave me, it has moved.
Troi: Worf, I hung that painting there at your birthday party, remember?
The painting has changed! Don't you see it?
Your hair, and your clothing, they have changed as well!