It's possible that:

1. The last shard sends to the coordinator a write indicating that it has finished the resharding operation.
2. The write gets lost due to a network error.
3. The shard sends the write a second time, which the coordinator applies, finishing the resharding operation.
4. The first write from the shard finally gets sent, triggering the invariant that the resharding instance for that UUID still exists.

max.hirschhorn proposed two solutions:

1. Have the donors and recipients use retryable writes so that the shard doesn't attempt the write again if it's already been applied, and
2. Have the donors and recipients use a precondition that will make the write a no-op if the write has already been applied.