This is a common idiom in our current codebase:
couch_config:register(fun("couchdb", "os_process_timeout", NewTimeout) ->
It proves problematic for hot code upgrades. The issue is that the anonymous fun is held by the couch_config process but belongs to the module in which it is defined (here, couch_external_server.erl). If an expert user loads a new version of couch_external_server, the anonymous fun will belong to the 'old' version of the module. If she upgrades the module again, couch_config will be killed. That's no good.
The solution is to export the function that handles the configuration change or db update event. For instance
config_change("couchdb", "os_process_timeout", NewTimeout) ->
I have a patch for this that we've been using for some time in Cloudant, just need to clean it up and apply it to trunk.