I often get double meat, too, which in principle costs extra, but that information has to propagate down the line, and I'd say about 1/3 of the time it gets lost. What remains is an ethical dilemma—do I inform them that they're under-charging me and thus pay for the extra meat, or do I keep mum and get the free second serving as a bonus?

N.B. I knew about the fajitas hack—I always get vegetables in my burrito bowl.