Roll problem

[GJRaidr@aol.com]

recently ive parked my car in my school parkin lot (i have a '92 Sentra SE-R)
and twice its rolled backward down the hill.. its not a steep hill at all
either...and both times i had the car in gear and the ebrake fully
engaged.luckily both incidents have only caused me a flat tire... tonight it
happened and the odd thing is when i tried to push the car it wouldnt budge
at all in either direction. why did it roll? i tried it again a couple of
times, and it didnt budge.. anyone have any ideas?

Gary Justin

92 Sentra SE-R