MTB Bent Cog...

[akb]

I must say I can't think of any reason why the teeth of the innermost ring could be bent inwards by the chain coming off...

That's my thinking. I don't crunch gears, ensure I give myself enough time to change gears so I'm not changing whilst pedalling hard. I honestly have no idea how this happened.

 

[fossyant]

Has the chain ever dropped off the bottom chain ring then jammed?

 

[BimblingBee]

Looks like wear and tear to me. Quite possible that it's been bashed on a rock, stone, log etc. I imagine a bike shop would (rightly) refuse that as a warranty claim.

Have a look about and you'd easily get a replacement for a tenner delivered. Superstar components do some patterned copies that are good.

 

[RecordAceFromNew]

That's my thinking. I don't crunch gears, ensure I give myself enough time to change gears so I'm not changing whilst pedalling hard. I honestly have no idea how this happened.

But as Fossyant said it is well out of warranty and it will be hard to tell what caused it. You could either bend it back with an adjustable spanner, or get it replaced. For the latter this is an absolutely amazing deal imho, and if necessary (depending on the spec and state of existing bb) in combination with one of these to match your bb shell width and in 110mm (for 47.5mm chainline) or 113mm (for 50mm chainline) to match your existing setup.

 

[akb]

Has the chain ever dropped off the bottom chain ring then jammed?

I think that's what happened when the chain came off. It certainly felt that way.