Temporary mapreduce colelctions are not dropped when they are done being used. Furthermore, it is not possible to drop these collections manually as it just returns "false" without any reason.

This eventually leads to this error on the secondaries in a replica set environment:
error: hashtable namespace index max chain reached:1335

This eventually snowballs and the secondaries are put into recovery mode without warning, leaving only the primary node up and the entire shard inactive.

I realize you can increase the nssize argument at the command line but it would only be a temporary solution. I've attached the logs from one of the secondaries.