Example. If I go out and drink 3-5 beers on Friday it's going to be -2lbs on Saturday (water loss) and then +2lb of FAT by Sunday. Doing quick calculations; 5 mugs of cheap CASS Korean beer is 750 calories, 91g of carbs. Probably more for "good beer". If you are already eating 100-150g carbs (Mark's weight-maintenance zone) then jumping to 240g on Friday would explain the fat gain.
Beer is definitely a cheat, but sometimes necessary. Thinking of switching to expensive whisky.