I got once a panel-mounted GSP failure - a GNS430 - due to a bad cabling. We could fly Geneva to Biggin-Hill IFR, using it, and a couple of days later, no signal. Hopefully, the leg home was VFR (no need for an certified GPS), and we had a couple of handheld devices on board.
Steam failures ? I got once a "heading bug" failure. As the HSI was turning, the bug was sometimes stuck, making its heading change, making the heading and interception modes of the autopilot unreliable.