What kind of tranny? If you can drive it and it's not making any grinding noises then probably the worst you did was tear some clutch material from the reverse clutches. You might want to drop the pan and see if anything is in there.
You probably figured this out by now but you need to fully stage the car THEN engage the transbrake. Are you running any electronics? My Dedenbear box has a lockout feature that can be set for a number of seconds so that once the transbrake has released it cannot be re-engaged until after the lockout period. I have mine set for 6 seconds and prevents engagement during the run if by some chance I accidentally hit the button. I saw a car wreck at my local track once and the assumption was that he hit the transbrake button at 100+ mph. It wasn't pretty.
George