mongodb/docs — pull request #1019 by joeytwiddle (2013-05-29 19:14:05+00:00)
Title: Corrected double-negative

It looks to me like {$ne: t._id} specifies we can only perform this update if t._id is NOT already in the pendingTransactions array. The original description seemed to be saying the opposite (prevent if NOT, instead of perform if NOT)!