When cats randomly go back to their kitty instincts. Often when you pet them and they are being adorable, then BAM! it grabs, kicks with their back feet and bites your hand! All when you thought kitty was being nice to you!
It may also happen randomly during the day or at night when they take off running for no reason or attack your shoelaces or (insert random object here) so fiercely you dare not move or you will be ripped to shreds.
Person A: "Wow! I thought I was finally warming up to your cat Angel, rubbing her tummy when she just randomly bit me!"
Person B: "Yeah, you gotta watch out for her. She can get into kitty mode pretty quick."