This happened to me not so long ago... on my beater fix, I had forgotten to tighten the chain in like a month... coming down the williamsburg bridge path, hit a bump and lost it. No hand brakes. Moment of panic. Then I remembered, you jam your shoe between the rear tire and the brake bridge (or whatever you call it when it's not drilled for brakes). I managed to slow down enough to make the turn at the end of the path, came to a stop and put the chain back on.

I check my chain tension every time I ride now.