Resharding's oplog application is multi-threaded and is susceptible to write skew under snapshot isolation. Resharding does a trick of writing an unreplicated "no-op" to a common document to get a serialization.

However there are cases where this no-op write becomes the only write in the transaction, leaving it untimestamped. This can lead to a mixed-mode update chain in WT that is problematic with durable history.