What can cause this? Chain caught between frame/ring underneath...

[donrhummy]

I was going up an ascent when I went to shift from the small ring to the middle ring and the derailler failed to move the chain. Instead, even though it was a full "pull" it just "trimmmed." So i dropped the derailler back down to the small ring and then as I went to shift back up the chain got wedged between the fram and the small ring underneath the bottom bracket. I have no idea how it got there.

How does this happen? (I need to know so i can fix it and make sure it doesn't happen again) It got wedged so badly that it completely bent/twisted the chain link and scratched the heck out of my frame.

 

[ttopaz]

Derailler has 2 limiting screws. One for limiting high gear, one for limiting low gear. Make small changes with them while checking the limits and still able to shift to the gears.
You can read more about this here: http://www.parktool.com/repair/readhowto.asp?id=75.
It is in Adjusting L / H screw parts.

 

[cruisin]

You got a thing called chainsuck.
This can be caused by trying to change under too heavy a load, a dirty chain,a twisted chain or worn components, or a combination of any of these.
It happens when the chain doesn't come away freely from the ring you are changing from and is then pulled up between the stays and the chainrings.