I received an ATC notification during one flight that my altimeter was
1000' off. They actually had me turn off the altitude encoding.

This, by the way, occured shortly after having my plane in to the
avionics shop for some repairs. Turned out, when they put one of the
instruments back in the panel, they introduced a kink in the static
line. I replaced the kinked section with a new hose and everything
worked fine...