On a good used copy of Yes Fragile LP I just purchased, I heard something really strange:
I had on my AKG 702s, fed by my Raptor headphone amp, VAC preamp and a killer analog front end and was listening to one of my favorite tracks, Mood For A Day, which I've heard 100's of times. It sounded beautiful and was completely involving... then, about a minute into the song, I heard a faint click and suddenly it sounded like someone turned on another mic, or inverted the phase, or something, but it was instant and dramatic! It was as if I was A/B-ing two different systems! Different soundstage, presence, one more enveloping, the first more laid back... weird! It lasted until just a second or two before the end of the song.
Of course my first thought, as is probably yours, was that something changed in my system at that moment: tube failure, some circuit that suddenly came on or switched off, etc. But no, I played the track again and it happened again at the same exact spot!
So I pulled out another copy of the album, also from 1972 (but a different pressing), and listened to the same cut without making any adjustments... it played flawlessly. No phase changes or any anomalies whatsoever.