Also may occur during update - haven't dug in.

When a deadlock exception is thrown in the DeleteStage, the stage is not restored before reusing via DeleteStage::restoreStage - this can allow a primary stepdown to go undetected or undetected cursor invalidation and is generally undefined behavior. Also, the DeleteStage does not store the working set member (WSM) when the problem occurs, so on resume the child stage will return the next member.