This is hypothetical, we haven't tried to reproduce it.

The scenario looks like this:

1. Collection exists and is unsharded.
2. Mongos attaches shard version UNSHARDED to the request.
3. acquireCollectionWithoutLocks checksShardingPlacement, it is correct.
4. Collection becomes sharded from some other client running shardCollection.
5. acquireCollectionWithoutLocks opens the snapshot at this point.
6. Collection is dropped from some other client running drop.
7. Collection is created again as unsharded from some other client running create.
8. acquireCollectionWithoutLocks checksShardingPlacement again and everything checks out because the request was sent with shard version UNSHARDED and the collection is (again now) unsharded.

Currently we have a check to prevent this exact scenario described in SERVER-62457 in the AutoGetCollectionForLockFreeReads case, however it might be broken, see SERVER-76559.