Took me a while to understand this, if I did. Lets see: In the event that we are ordered, then we know that the error info for op that caused it has been enqueued, so that is the right one to tie in the exception.

However, shouldn't this be 'back' rather than 'front'? Don't you do push_back on _writeErrors?

And, what if we have write concern errors? You probably already answered this in an older review, but remind me :)