Thats what happened to me last weekend, stuck in 3rd, shifter felt funny. Took off clutch cover, hook on end of shaft had slipped off the shift star pins and jammed. Not so sure about the washer fix, there's not much clearance for the shaft to move side to side. The plastic spacer on the shaft keeps it from moving too far to the left. A boss cast into the clutch cover keeps it from moving to the right. The only clearance is the thickness of the gasket.(maybe 1/32"). That's not enough to cause the problem, the hook is about 3/32".
The arm on the shaft must flex enough to let the hook slide off the pins. A friend with a history of breaking stuff was riding at the time, but he was wearin sneakers, not boots!!
Anyways, heres what NOT to do: DON'T try to bend the arm or hook!!! I thought i'd open the hook up enough to get to the middle of the pin, instead of just barely catching the end of the pins. Just started to bend when, CRACK !, oh hell, guess I'll be ordering a new shaft tomorrow.
Maybe the new shaft will catch the pins better, but I doubt it, sounds like this is a pretty common problem.