actually, it happens alot....

when you get a bent shift fork, or alot of wear on the shift drum.

sometimes though, you just get stuck on the point on the shift star (man, that's hard to explain, so just trust me) and you get what they call a "false nuetral". that's not really shifting itself though, more like getting stuck between gears when your shifting.

that, or you didn't fully click it into gear and the shift star just rotated backwards, but that would only happen if you were in 2nd gear, and it flipped backwards into nuetral.