Jeff Niemann does not weigh 280 pounds, but he used to. When Peter King and I saw him pitch a couple of years ago in Indianapolis, King asked, "Jeez, who's the defensive tackle on the mound?" By the following spring, however, Niemann had remade his body with diet and exercise; I literally didn't recognize him, though at 6-foot-9, I should have had an easier time figuring it out. He now looks more like an NBA forward than an NFL tackle. I'm no carnival guesser, but I would have spotted Niemann at around 230, probably a bit low due to his height. This year, the Rays weighed him in at 260. The team's trainers weighed every player as part of his physical, then rounded to the nearest five.
The problem, however, is that the Rays' media guide still lists him at 280. So does BaseballReference.com. ESPN was the only major source that has the correct weight, though it's unclear when it was changed. That's not a big deal if you're just looking up some info on Niemann or any of the other players in the major leagues. However, if you're trying to project player performance, it is a very big deal indeed. PECOTA, Baseball Prospectus' projection system, uses height and weight as one of the components in comparing players.