Well, to clarify/correct myself...

The scale can be wrong, (read different masses incorrectly) but it should always return to zero when the different weights are returned to zero. Unless it's been moved, zero will always be zero on a beam scale. (Everything else might not read correctly as you go along the beams, but ZERO is ZERO and you are comparing the same mass both times, now or 30 mins later or 1 month later).

Sometimes there are magnetic fields around the bench that will interfere. Some lights can cause this. But I bet you dollars to donuts your scale is moving around on the bench.