I guess it just never happened to me. All the gates I saw were for side material, not down necessary paths. If anything, as a talking character I often had several parallel options for proceeding down the math path.
It happened to me. I forget the exact details, but I believe my pure talker merchant was on his way to rendezvous with the ruler of Maadoran at the endgame temple, only to be stopped by some blockade or other, which came completely out of left field and invariably led to a combat encounter I had no chance of winning.
I ended up having to ally myself with the ruler of Teron instead, whom my character had never met and had no motivation to support, which circumvented the encounter somehow (possibly the enemies I'd encountered belonged to house Teron, but what earlier decision had led to the existence of this event I don't know). As far as I could tell, this was my only option if I wanted to see an actual ending for that playthrough. It felt mighty cheap to have my character play Grima amongst the Maadoran elite for most of the playthrough, only to be stopped dead in his tracks by a couple of unavoidable meatheads with sticks right at the end.