Pansies...especially yellow or purple ones.
Dark colored splotches in the dirt. At the time I ride one corner of the arena is in shadows. The tree nearby is starting to leaf up so the shadow of the branches hits the arena and in a breeze the leave shadows are moving. A second part of that is that the large arena lights make these round splotches on the ground..we dance around it sideways for a while. Also, after the guys have watered the arena with the big firehose, of course you get some small areas where the dirt got hit too hard and it makes a little wall..about an inch or so high. We won't walk through those either and we stop two walk strides in front of them, drop the head to snort at them and then back up.
Trucks, backhoes, trailers, kids running around and screaming, no problem...but that dark dirt is going to swallow us whole.