Kids riding bikes without helmets.  Yes, I know we all grew up not wearing them, but that doesn't mean it was the right thing to do.   As a kid, I ended up in the Emergency Room one time for a crash that would have been considerably mitigated if I had been wearing a helmet.  I was in a ward with three other kids that crashed that day.  I was the only one without a concussion, and one of the kids was projectile vomiting.  That's a sound you don't forget.  These days,  I'd never consider getting on a bike without one. 
Because helmet use under the age of 16 is a law, when I'm on duty as a volunteer with the local Park Police, I ask parents where their child's helmets are.  I get some crazy excuses.   Some of the best:
"Oh, it's still in the car"  - Ok, so you made the effort to bring it with you, but not put it on? 
"He says it's uncomfortable" -- Lady, have you ever had a concussion?   That's a lot more uncomfortable than wearing a helmet.   
"Oh, it's a law?"  - no comment
"But he's on training wheels" --  Bikes on training wheels can fall over, too.   Better to get good habits started early.   My daughter knows she has to wear her "funny hat" any time she gets on a bike, because that's what we always did, from dragging her in a trailer to riding a tandem bike with me. 
Or they have the helmet attached to the handlebars -- what good is that going to do?   
But maybe the worst are the ones where the parents are wearing helmets and the kids aren't.  Or one of the kids has a helmet and the other doesn't.  "Oh, you must love this child more than the other..."  Or when I see a couple going out for a ride and the boyfriend is wearing a helmet while the girlfriend isn't.  I guess chivalry is dead. 
That's my two cents for Pet Peeves