there's many a poster i did in the past where i (and the promoter) thought they we were doing the official poster. then suddenly, there were a half dozen other posters - some great, some sucky - showing up. the bands let all their friends do them as well. and, when all those other posters are selling in the score and you can't sell a single one of yours, it sorta hurts.
sometimes promoters get all excited like little kids and give poster jobs to everybody that walks in the door. they think like they're doing everybody a big favor that they benefit from by getting lotsa free advertising or something. they never seem to notice that they're sorta stomping all over the individual artist and their hard work. it's sort of a slap in the face, a 'belittlement'. makes your effort seem like it was unappreciated.
and then sometimes people just do them without asking.