It happens more than you might think. I think this happens mostly with shops that don't do a lot of trimix and don't have a good system for charging. I once worked in a shop that did very, very little trimix, and it did not have a booster. Consequently, filling over a partially filled tank was problematic. They would have to bleed off at least some of that expensive left over gas in order to make the fill.So if you show up to the shop with your tanks half full of 18/45 looking to get topped off with more 18/45 they charge you the same amount as if they were completely empty? That's bananas.