Scary title!

At a stop light I had trouble getting down to 1st.  I let off the clutch and the bike was in neutral with no neutral light.  Finally I gave a bit of gas and rocked it forward and stomped down on the shift lever.  I got first gear with a pop, and heard what sounded like a broken tooth or sheared pin rattle around inside the engine a couple of times before dropping down into the oil pan.   

That is what it sounded like anyway. 

Bike shifts and sounds fine now.  Has over 1/2 full mark of clean oil.  Bike was not hot.

Any ideas what happened or is happening, and how I can stop it from happening again?

Thank you for any ideas.

Loose chain
Cool.
Buddha.

I would not call my chain too tight.  1.3" slack on the side stand.
I guess that can explain the pop.  I do not know how it explains a pebble in a coffee can. 

?

Dont rev before shifting. Rocking back and forth will do it, in fact GS behaves better if you roll back than forwards.
Cool.
Buddha.

Buddha

I will take your advice if I have trouble getting into gear again. 

Do you know the mechanical cause?
Do you know what can be done with a wrench to fix this?  Is there something worn or broken that causes this?

Thank you

Mechanical cause ... lets see you rev up, and the engine side of the transmission now is revving @ approximately the speed of the crank/2, the wheel side of the tranny is at lets see ... 0 ...

With a wrench, undo the hand that revs the bike before shifting. Just remove that hand with your wrench.

Worn or broken - well it soon can be if you dont stop doing it.

Cool.
buddha.

you shouldn't rev the bike, then put it in gear, a slight rol backward is plenty most of the time.  and it is better to be in gear at a stoplight so you can get out of the way incase someone talking on their cellphone doesn't notice the red-light

OK, OK, OK,
I swear on all that is holy I will never rev the engine and do a neutral drop again.

NOW, with that out of the way...

I wonder if I should pull the oil pan and look for a little piece of metal.  Mechanically.  With a wrench.

You can dooooo eeeeet! J/k

You should do it. Take pictures, and let everyone know what you find. It might help someone down the road. You never know. 

In the pan its out of the way ... it wont make its way past the oil pick up screen and the metal will sit till you pull the pan anyway.
BTW you also didn't have a hunk of metal flying about in there like you imagine it, cos if it did ... you have a nice new paperweight.

Gears engage on the shaft with the gear to their left or right via a set of side ways teeth and corresponding holes. There is 3 teeth and 3 holes to match in the wall of the gear. These are referred to as dogs. These in your case didn't drop into their slots for a few rotations. So I doubt you lost any large quantity of anything, you lost a few 1000's off these things and egged out the edges of the holes.

So the wear you would have got in 10K miles lets say, if you were riding it like the buddha does, you got it at that stop light ... of course after the buddha is done with stuff ... transmissions and clutches look like new ... cos I never ever ever ever miss a shift or even rev @ shifting. In 40K miles on my 89, I missed a shift ... exactly 1 time. I also never clutchless shift and never have I done what you did. Even with idle hovering, I manage to avoid your situation bu loading the thing letting the revs drop due to load and then quickly clutching and shifting. Came close to lugging it, but never did that either.
Cool.
Buddha.

I know I wouldn't sleep well at night if after what you describe as a piece of metal rattling around in the case happened to me! But I will defer to Buddha he's forgot more then I know about bikes. If you had the time though I would pull the pan the next time you change oil just for a sanity check. 

Robert

THANK YOU.  THANK YOU.  THANK YOU.

If I pull it, pics will come.

In the pan its not hurting anything ... I keep some spare change there, the extra shims I may need on the longer rides, the valve tool, the allen wrenches etc etc ... things in the pan ... stay there unless you tip the motor on its head ...
Pulling the pan will reveal a ton of junk, lacquer, silicone, metal crap and pretty much anything else ...
Your motor is a 2 storey motor ... oil  pan is first floor. the transmission and crank etc are second floor. The rods and pistons are third floor and the head is the 4th floor.
Only way to get anythign from 1 floor to another is to use the one and only staircase. AKA, the oil galley. The bottom of that beauty is a filter that wont let anythign larger than a grain of sand through. Then it gotta get pumped and the thing first runs into the filter and then gets to anywhere else. So Fu[k you hunk of metal, you aint getting anywhere near anything important.

Cool.
Buddha.