This only happened once and I havent been able to replicate it.
In NDisles, I took a Panzer IV and drove uphill *on the resupply road* southeast of the base. After a certain distance uphill in 2nd gear, the tank stopped and could not traverse any further. The grade was not too steep to traverse. I tried turning around and when I did, one track was off the road and it rolled off the side and fell into oblivion. I alt-F4'd as I was unable to .ef or bail.
winXP
1.2ghz
512mb PC2700
9600xt
dx9b
Cat 4.3s