I've got a mountain bike which occasionally drops the chain off the lowest gear, and then the chain gets stuck in the bottom bracket. This happens while pedalling.
Once the chain is there, it takes either a lot of pulling (potentially bending the chain), or removing the crank to get the chain out.
Either way, it's a pain when it happens on the trail.
Note: I'm not talking about chain suck.
And I've seen products like the Deda dog fang chain watcher and Jump Stop, anyone have any experience with these?