Originally Posted by
cmolway
...Pushing hard on the pedals at the first light, I felt my chain "slip". It didn't skip, but rather slipped on the cog. My chain is at the end of it's second season so I attributed it to that. A mile down the road, at another stop, I again jammed hard on the pedals only to have it slip again, followed by a crunch and a locked up rear wheel.
...
My rear derailer was sheared in half.
Ouch. Pics or it didn't happen, of course.
Slipping on a cog can indicate that the cogs are worn. Check that the cog teeth don't appear too asymmetric, especially on the cog you use the most. If they are worn, a new chain won't fit them too well.
But a busted derailleur? I want to see that. You run indexed, right?