One cold, snowy January Saturday night a few years ago we won the last race (went off a little after midnight) and the horse would not piss. Absolutely had no interest in it. This was a horse who would normally piss within 5 minutes of getting to the spit box.
Not this night. Tried him on straw, tried him on shavings, whistled to him forever, walked him around the completely empty paddock, he drank 2 full buckets of water in 2 1/2 hours before he finally peed. Luckily we didn't have to wait for blood, because this was when they were doing the random pre-race blood tests. Didn't get out of there until almost 3, and it was light out by the time we got home.
The State people were very understanding - you could tell they'd been through this before
Kinda ruined Sunday, but we won a race so I'm not complaining!