It happens.

Also, if 50 BBs is upsetting you (I totally understand, I play $5.50 STTs and get upset when I lose 5 buy-ins), try not looking. Buy in for some really weird, high number (I dunno what stakes you play, but if it's 10/20, buy in for like 6845 or something) so it's hard for you to insta-calculate your session's profit/loss, and only look at the cashier at the end of the month.