SERVER-48067, I measured performance by building a unique index on a standalone on a field that was identical in 2 million documents. One of the times I ran this build, it hit this invariant with a WriteConflict. The exception appears to have occurred sometime after inserting keys from the external sorted into the index, which was the last log message printed before the invariant was hit (with default log verbosity). Having a stress test here (probably either a standard jstest or an FSM jstest) will hopefully help reproduce this and identify where the WriteConflict escaped from. Note that I specified "a unique index with a large number of non-unique keys" in the title since that's the scenario I ran into this issue with, but it could be unrelated to this specific configuration.