I was riding my new to me 2004 Giant Boulder. I was happily pedaling up a hill and shifted from the big chain ring to the middle chain ring. I pedaled some more and all of a sudden my pedals locked up. I looked down and the chain was wedged behind my front gears and the frame (chainstay?) I got off and saw that the rear derailer tensioner (I think that's what it's called) was also pulled almost horizontal.
I loosened all of this and saw my frame was scratched in numerous spots down to the bare steel!
Anyway, I rode a few more pedal strokes and it happened again, scratching it more! I got off fixed it and turned toward home. It did not happen again, I shifted a ton just trying to make it happen and nothing.
First, what happened? And second what can I do to make my frame not rust in those spots? Do I need to worry about that? This will be my commuter bike and will see a lot of use.
A picture is attached illustrating this.