This might be incredibly obvious to a lot of people, but I often see people making the mistake of thinking that if you gain, say, 10lbs, and your bf% stays the same, then you didn't gain any fat.

Let's think about this mathematically, though. If you're say, 20% bf at 200lbs, then you have 40lbs of body fat. If you bulk up and you're still 20% body fat at 220lbs, then you actually GAINED 4 pounds of fat. You did not "stay" at the same amount of body fat, it's just that you gained enough muscle to keep the ratio of fat to lbm the same. I see this fallacy mostly with people who think that they're losing fat while building muscle at the same time. It's a pretty nit-picky thing to point out, but can still be fairly significant depending on the percentages of bf you're working with.