PHP-1228 describes an edge case with the 1.x driver's method for removing files:

Because this isn't a transaction, it is possible that a new file gets inserted while another process is in the middle of MongoGridFS::remove(). And if that new file happens to match the removal criteria, we could end up deleting its fs.files document and leaving its chunks behind. That said, it looks like you were only removing fs.files documents in your OP, so I don't think this would be an issue.