Always be aware of a subtle failure, many moons ago doing a DME arrival and was tracking on the VOR perfectly until I got that funny feeling that it wasn't right. Giving the CDI a turn on the HSI, showed we were on track whether it sat, going back to GPS mode on the HSI likewise spot on track, wasn't until I looked at GPS receiver and we were about 15 degrees of track. The VOR was working okay, ATC confirmed that no one else had any issues.
The reason for the failure in the HSI CDI was a relay that swapped the VOR input for GPS input had failed in GPS mode, therefore no VOR or GPS information was getting to CDI.