I've not had the problem at ATG, but "THAT" aftertaste was one of the reasons I stopped going to O'Neill's of Suffolk St. - I had three pints in a row (a pale ale, a red, and a nitro stout) that where all seriously marred by what I assumed to be unclean lines (although not fully sure) - barstaff weren't bothered in the slightest when I brought it up with them.
I'd say the lads at ATG would want to know and would investigate it thoroughly.