#1 was back when I was working at Funcoland in New Brunswick NJ, an older woman brought in an Atari 2600 and a box of carts, asking if we repaired 2600's.
All it needed was a contact cleaning, which we did. She was so happy that it worked that she offered up any one cartridge from the box.
My friend dipped in and said "that one" , to which she gladly obliged, and left.
When he turned around and held up a copy of "Chase the Chuck Wagon" my eyes nearly fell out of my head.
That tops the list, and the others don't really have much of a story...
#2 was probably getting the Vectrex light pen boxed, and the art and animation carts all in one "lot" purchase
#3 was a copy of Crazy Climber for 2600, which was gifted to me from a friends collection - again, for getting his Atari hardware working.