I ran into a strange one the other day at a customer with a VMC (Fanuc 21iM).
After power up, he handwheeled X, Y and Z close to the fixture and started the spindle to use an edgefinder. At this point all was well.
He then moved Z down and X over. At some point, he noticed the Relative and Absolute position displays were not changing along with the Machine position!!!
I paid a visit to the customer site and witnessed this myself.
Has anyone seen this before?