I think my scales are faulty

I have a tendency to do multiple weighings in one go and I think I've fucked up the scales.  They're digital, so pretty accurate - at least they used to be.  I don't know.  They're not really mine, they're my parents and they're in their room so I can't usually weigh myself whenever I want.  I'd get some of my own but where would I hide them?  I wouldn't want to have to explain why I wanted my own scales.