Same thing happened to me with a cheap light meter I got off eBay years back. I ran a whole roll of film through it testing different speeds and every shot came back either blown out or dark as night, but the meter swore everything was dead on. Turned out the thing was reading two stops off across the board and I had no clue until a guy at the local camera shop checked it against his known-good meter. Cost me a whole shoot and a bunch of good shots I can never get back. Trust your gut over a sketchy tool every time.