In reply to johncoxmysteriously:
I have carried a small adjustable spanner around in my ropebag for a year for this reason, and have used it precisely once.
I was worried about over-torqueing bolts and breaking/weakening them, until people who know what they are talking about (e.g. Jim Titt, Ian Lloyd-Jones) said with a small spanner and a bit of common sense that's really a negligible risk.
I suspect if the rock is more or less vertical and the bolt is more or less horizontal then the direction of load on the bolt is still going to be pretty much ok even in the hanger is loose. I've seen one case where the bolt was drilled at a right angle to very overhanging rock, and a loose hanger had pulled it out an inch and bent it thirty degrees. Decided that was not a good place to be projecting at my limit.