When I first started playing music (Melodeon) I bought a metronome(mechanical in those days) to practice keeping a steady time (playing for dancing so fairly important).
To start with I was, in all seriousness, totally convinced that the metronome was slowing down. I'd stop mid tune to wind it up (I know that that doesn't actually affect the speed), and more than once considered taking tack to the shop for a refund.