Consider the following sequence of writes and a collection A:

// Checkpoint T= N

// Checkpoint T= N + 1

After SERVER-120541, we will remove the fast count entry for A as soon as we drop it. However, when we are calculating size and count deltas from the oplog while flushing at Checkpoint N + 1, we will see inserts for collection A and upsert an entry for collection A. This entry will then remain in the fast count collection.

We can avoid this by only deleting the fast count entry for a collection as part of flushing on checkpoint.