A classic one on last saturday:
Call: illegal formation (and also added with "to many in the backfield" over the mic)
HC: Which number?
Along those lines, had this in a youth game (11/12 yr olds) this weekend.
First play of the game. Two wideouts on my side obviously off the ball. Two running backs and a quarterback. So fairly easy call. I'm on the visitor sideline, and they see it immediately and want the flag out "That's an illegal formation! That's an illegal formation!" I tell the excited coaches it's not till the snap.
So they snap it, I toss the flag, they gain about 30 yds, the visitor coaches are all happy knowing it's coming back...and the home coach across the way is livid. I know him from previous dealings as he's a league asst director.
Coach: (after angrily charging 10 yds onto the field) "<my name> WHAT"S THE PENALTY? WHAT'S WRONG??"
Me: (considering telling him to get back, but since I knew him and there's no wing over there - 3 man - I just yell back) "Mike, illegal formation, both the wide guys were way back off the line"
Coach Mike: SO WHAT?
Me: Well, 2 receivers, 2 running backs, and the quarterback - that's 5 in the backfield
Coach Mike: WHAT'S WRONG WITH THAT???
Me: <blink, blink> You can't do that.
The entire sideline behind me is just laughing at him and half the crowd is too. They line up for the next play in the exact same formation and he angrily calls timeout to fix it.