3spd tranny drain plug?

alright i went to change my tranny fluid today had the 10mm allen socket and tried to take the drain plug off and it wouldnt budge the threads that the allen socket went to twisted with the allen socket after a little force i ended up just dropping the pan but now i gotta do the differential (seperate in the 3spd) and was wondering what i did wrong on the first one so I dont do it on this one? any help would be appriciated thanks!