The rollback code creates and stores pointers to memory that is not guaranteed to still be alive here: https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/rs_rollback.cpp#L242

At that call to insert, fixUpInfo can potentially outlive the data that doc points to. doc points to data from a query, which happens to always be alive in MMAPv1, but that might not be true for other storage engines.