The rollback fuzzer caught an issue when doing data validation cross a replica set that rolls back (an instance of RollbackTest) some operations and a "control group" replica set that does not roll back, there seemed to have been an issue where a temporary collection only exists on the replica set that did a rollback.
The "control group" replica set has 2 nodes to avoid differing behavior wrt. temporary collections one a standalone or single node replicates.
This suggests there's behavioral difference between oplog application vs commands performed directly by a client.
The failure is reproducible by downloading the js file attached to this ticket and running ./mongo --nodb rollback_test-convertToCapped.js
After chatting with Judah Schvimer, I'm filing this ticket for future reference, but it may not be something that needs to be looked into in the immediate future.