I had a kermesse yesterday, and since it hadn't rained here since July 2 (really) I thought I'd be safe running the brakes close to the rims. Unfortunately, I had them even closer to the rims that I meant to and by the second lap of the race I had somehow collected enough dust somewhere in my braking system that the front brake wasn't releasing from the rim at all.
After struggling through a couple of laps that way I pulled into the pits and tried to unhook the brake to clear it out, but it would budge away from the rim. Someone who was racing later in the day helped me out and freed the rim enough to get it to spin temporarily, but within half a lap it was rubbing again.
Who knew that dust clearance could be an issue!
Lesson learned: aside from the obvious lesson of not doing a bozo job on the brake set up, I learned never to go into a race with my barrel adjusters tighened down all the way. A few turns of the barrel could have saved my race.