Once you have stated a joke or "pun" and nobody noticed you even talked, the person sitting directly beside you repeats the joke and everybody heard it and he/she gets all the laughs and all the credit for your unique joke.
(You) "Why can't Helen Keller drive a car?...Because she's a woman!!!!"
(Crowd)-----(Didn't acknowledge your existence)----
(Pun-Stealer) "Why can't Helen Keller drive a car?...Because she's a woman!!!!"
(Crowd) "HAHAHAHA!!! He's so funny, if only I were as awesome, unique, witty, and brilliant as he is! I bet all the girls are begging to sleep with him!
(You) "I HATE YOU, YOU FRIGGIN' PUN-STEALER! I WILL EAT YOUR FIRST BORN CHILD!"