My favorite BC was the one at Belmont in 1990.
Only time I ever made a huge score with a mistake ticket.
The OTB's in Ny state used to use a letter system, rather than numbers. And the letter system was constant and never tinkered with, until that day, unbeknownst to me.
I made a 10 dollar 3 horse exacta box on the Classic and went to a friends home to watch the last two races. I realized while there that they had tinkered with the letter system on that day and made the entry A and A and the rest of the numbers altered accordingly.
Instead of having Unbridled and his entry mate with two other horses. I found out that I had Unbridled and his entrymate with two other horses I didn't want.
One of them turned out to be Ibn Bey, who ran 2nd at around 40-1.
I was in college and the 3 grand or so that I got back represented a huge amount of money.
Sadly, thats the last score I ever made with a mistake ticket.