My weirdest on-track experience: During a steeplechase race in college, as I was coming off the first turn of lap 5, a hammer--the hammer-throw kind--hit the track near the curb right next to me. Its handle actually whipped under my left foot, and the hammer bounced and slid along beside me for about 15 yards. I started yelling at the world in general, "Hey! Hey! Did you SEE that?!" and then I had to deal with the next hurdle. Almost as weird: after I finished I didn't even remember the hammer for a bit, and then I did, and I absorbed it more thoroughly and thought, "If I'd been one second slower I might have just been killed in the middle of a race."
Worse luck: A teammate of mine was knocked semi-conscious and given a concussion when a soccer player blasted a ball into the side of her head as she ran intervals on the East Sixth Street track in New York City.
Maybe scariest of all: another soccer guy ran off the field and collided with another woman on our team on the same track. She fell down, and her husband (world-class master guy, 68 years old, probably 130 pounds) went up to the player (big young guy) and demanded that he apologize. He got loudly defensive and wouldn't do it. The runner told him that he had no class. The guy stormed off across the field saying that he was going to "get my gun." The old runner stood his ground, saying, "I'll wait," while 20 of his teammates desperately tried--and finally succeeded--in getting him to leave. The guy didn't return, at least while we were there.
There's basically no time on any New York City track, except closed-off college ones, when you aren't going to have to deal with the more-typical obstacles like strollers and benches in Lane 1 and flying objects of all kinds. No one likes it, but we have to deal with it, and it's still better on the average than trying to measure off road intervals somewhere--especially because we meet at night when it's dark for a lot of the year, and the track has lights. I started thinking of it as practice for staying calm while running hard in a race, so if weird stuff happens (hopefully no more hammers) I won't bail out mentally.