https://jira.mongodb.org/browse/SERVER-106075 reports:
A post-authenticationflaw in the network two-phase commit
protocol used for cross-shard transactions in MongoDB Server
may lead to logical data inconsistencies under specific
conditions which are not predictable and exist for a very
short period of time. This error can cause the transaction
coordination logic to misinterpret the transaction as
committed, resulting in inconsistent state on those shards.
This may lead to low integrity and availability impact.