The test removes a shard and tries to put it back again. However, it is possible that the migration cleanup task has not yet completed so attempting to re-add the shard will result to "local database exist in another shard" error.

The test attempts to prevent this by checking the currOp for the cleanup task:
https://github.com/mongodb/mongo/blob/r3.3.14/jstests/sharding/remove2.js#L26-L30

But the issue here is that the balancer can run with

and this will mean that the cleanup will happen inline with moveChunk and the check above will not catch this.