The MongoDB test jstests/core/txns/commit_and_abort_large_prepared_transactions.js is failing occasionally with a validation error, which is:

Error: Background collection validation was not successful against all replica set members
Index with name '_id_' has inconsistencies.
Detected 2 extra index entries.

The test cases started failing on June 10th corresponding to this set of changes being dropped from WiredTiger into MongoDB.

There is a similar failure mode reported in WT-6417 - though that test doesn't (obviously) involve prepared transactions, so I'm opening a different ticket.