createCollectionForApplyOps runs on secondaries and has functionality to temporarily rename a collection out of the way into a tmp collection, with the expectation that it will be renamed back again. This is for oplog replay when this scenario happens:

1) createCollection(foo.bar) w/ UUID 1
2) renameCollection(foo.bar to foo.baz) still w/ UUID 1
3) createCollection(foo.bar) w/ UUID 2

createCollectionForApplyOps will rename foo.bar w/ UUID 2 out of the way into a tmp collection when it sees 1) and create foo.bar w/ UUID 1, with the expectation that it will eventually see 3) and rename the tmp collection back.

However, this code can leak bugs. When secondaries somehow have a collection that the primary doesn't, and the primary then creates the collection and the secondaries rename their original collection out of the way, orphaning it.