Why does [my_document].save result in a 'collection already exists'; code: '48'?

When I run [my_document].save the first time it runs no problem, creates the collection and saves the document.

When I run it the second time (and any subsequent times), I receive the error 'collection already exists'; code: '48'.

I am running mongo 3.2.0 and mongoid 2.4.7.

If I run db.dropDatabase(); in the mongo console, then I can run it again without trouble...but just once. After that, it won't work at all.

I can also delete the tmp/mongodb/[my_collection] directory manually, and it works again...just once, and not again.

I've run brew uninstall mongodb and brew install mongodb more than once to no avail; same behavior.

Rather frustrating. Of course the collection exists. It should exist. And it shouldn't be a problem. Any assistance would be much appreciated.