I discovered this problem while trying to solve a problem with a Nagios MongoDB monitoring plugin that was returning "CRITICAL - General MongoDB Error: 'utf8' codec can't decode bytes in position 0-1: illegal encoding" while checking several MongoDB statistics from the serverStatus command.

See the attached output from serverStatus using a browser.

First note the interesting and large array of fields in "locks" and "recordStats". The documentation doesn't explain that. Note near the bottom of each field alphabetically there are several keys that appear to be binary and possibly badly encoded.

I'm supposing that these badly formed strings are breaking the python string codec in the monitoring plugin.

Restarting the mongodb instance resolves the issue by removing nearly all of the locks and recordStats.