At The Etihad ,one that really pissed me off was Boro at home in the FA Cup about 5 years ago. Think we played some meaningless friendly in The Middle East to 'develop the brand' a few days before. Flew back into Manchester night before (so I read) players were knackered and lethargic and we lost 2:0, that really irritated me.
Went to a couple of shocking 'performances' at Elm Park, Reading in consecutive seasons late 90s, lost 2:0 and 3:0.. 2 games that blend in to one lump of dog turd.
As mentioned elsewhere, Bournemouth 89, throwing 3 goal lead and promotion (temporarily) was bad. Also remember some complete racist knob abusing fellow blue leaving the Kippax because he 'looked like Luther Blissett'! who'd scored their 97th minute,(!!) equalizer!
Actual genuinely awful, pathetically shockingly inept performanc, a 1:0 home defeat to Notts County first time we got relegated really, really pissed me off (didn't quite prepare me for what was to come on the last game of season though!)