These tapes have just been sitting sideways in my cabinet - never in a car. At the point where this one got stuck, the reels were turning so hard that even fast-forward & reverse weren't working. The slapping must jolt something loose or free up the tension somehow, as afterwards the FFW/REV worked again. When I was recording the other side, I had to do it again, because it didn't get stuck, but the pitch started switching between low and high. I have a few other old tapes that do that with the pitch, which I thought were from the recording having degraded. But now I'm going to try this trick out on them too, to see if it fixes them too.