Say I start mongod with mmapv1 enabled in a clean directory:

mongod --dbpath=mongodata --storageEngine=mmapv1

Then exit the server.

then if I start the server with the same dbpath but with wiredtiger:

mongod --dbpath=mongodata --storageEngine=wiredtiger

the server will happily start writing data files to the same directory mmapv1 was using. (Also happens if I do wiredtiger, than mmap)

This is probably going to cause some confusion and/or misconfiguration at some point. I think it would be better if we detected that we are starting in a non-empty dbpath with incompatible data files and refuse to start.