What kind of error? Is it a tolerance error? Is there an error code indicated?
I used to use a GTS 802 that would, upon occasion, add in an extra 4' into the reverse H angle. The occurrence wasn't consistent, but the magnitude was.
If we got a clear definition of what caused the problem, I don't recall what it was. But we sent it in, Topcon fixed it, and it didn't come back.