On a sharded collection with two chunks (both chunks initially on the same shard), when I tried to move one of the chunks to the other shard and back again, the second moveChunk operation that would return the chunk back to its initial location seemed to be blocked on the deferred CollectionRangeDeleter operation:

(first moveChunk returns)
I SHARDING [Collection Range Deleter] Deferring deletion of test.t range [{ a: MinKey }, { a: 1.0 }) until T
(second moveChunk starts)
(at time T)
(second moveChunk returns)

Both moveChunk commands were executed with _waitForDelete: false.