-middle school/junior high boys who feel the need to make obscene/sexual comments as we run by... seriously, how old are you? I doubt you even know what you just said!
-cars that honk as you run by and startle you so you trip/run into something/fall of the sidewalk etc.
-somebody already said this, but walk/sprinters in a race... I haven't encountered any in HS since I've sped up a bit, but in middle school they always beat me b/c I actually ran the whole way.
-Cars that run red lights and almost hit our group when we're in the crosswalk.... WITH THE WALK SIGNAL!!!! this happened to my group once... but don't worry, no one was hurt and we managed to flip the guy off before he got away
ok, that's enough for now... I'll try to think of more.