Quote Originally Posted by laura* View Post
There might not be anything "wrong" with the parts. This can be a characteristic of 1xN drivetrains. When shifting to a smaller cassette cog, the shift can happen suddenly and fast. This may cause a sideways wave in the chain.
That's interesting, I'd think that with one chainring in front you'd be spared a chain ever falling off again. I never thought that the front derailleur would work as a chain guard. But 10 speed in back is a pretty wide spread for the chain to handle too. Park Tool (I think?) has a very neat and cheap little gadget called a chain checker btw, which tells you in a second whether your chain is worn. I love mine, and have recommended it as basic maintenance gear to many people.
Also the rear derailleur could be a little slow in taking up the slack if you've ridden in winter and it's a bit draggy, so that you have an instant where the chain has a shorter circle to travel but hasn't been pulled tight yet. Just an idea.