When you have sex with someone that you have had sex with in the past -- But it only counts if you have slept with someone after you go back on the list because then it wouldn't really be going back on the the list now would it.
Sleeping with an ex-girl friend after you have slept with someone else.
A movie cliché in which half the cast is killed because one of the characters, in a moment of imminent disaster, went back to save their dog, purse, sentimental trinket, or other such non-essential item. Often used by movie makers to ramp up the tension at crucial moments, but usually it ends up just looking idiotic.
"Look at her. There's a 1000 ft wave heading towards them at 700 miles per hour and she's going back for the dog."
"Okay, the frickin' world's about to end and these idiots are running around with suitcases full of their belongings. Talk about going back for the dog."