I would not go as far as disappointing, but in terms of not living upto my own preconceived expectations...
Yellow Tail Congo Tetras (Alestopetersius caudalis): Maturing males get really nasty with each other, often having group jousting battles, which produced massive flank injuries that eventually killed the fish depsite meds and water changes. Ended up rehoming my last few, in hindsight I think my group was all male after the only female died on the first night after being brought home from the fish store.
Pyjama Synos (Synodontis flavitaeniata): Supposed to be a very social syno, but my four "teenagers" (~6-12cm SL) are constantly convered in white bite mark scars, thankfully the wounds do not break the skin unlike the above! A little better now they are in my 5x2x2, more room to get away from each other I guess
Upside Down Synos (Synodontis nigriventris): Supposed to be very social, which they seem to be in the middle of the night when I've filmed them on my night vision camera, but I never see them out and about during daylight hours (either swimming around as a group or all "treading water" in one area of their 48x12x15).
Clown Syno (Synodontis decora): I've had a gorgeous ~18cm specimen for over 18 months now, probably my favourite fish in my 6 tanks in terms of colouration/markings, but he(?) is so reclusive for a fairly chunky fish! I only discovered they were a schooling syno a few months following ownership, so I jumped on the chance to buy 6 ~3cm youngsters last summer, which growing really nicely in my 48x12x15 and were ~8-12cm SL by February and just getting to the stage where I was seriously thinking about introducing them to the 5x2x2 to join my original specimen... But then my worst tank disaster to date kicked in, I lost all 6 youngsters to a really nasty/rapid case of Ich within a week of spotting there was an issue, along with Steatocranus casuarius "dad" (in one of my profile photos) and one Synodontis nigriventris (plus several Ilyodon xantusi fry, inevitable casualties when I was advised that I ought to hit this outbreak hard with full dose Protozin and a low concentration of salt). I still beat myself up about this regularly, nearly five months on. My single specimen still lives in the 5x2x2 community, but he might be moving to someone trying to rebuild a S. decora group in the near future, which for the sake of this lovely catfish I'm happy to do as I'm not sure I have the strength of character to try and grow on another batch of youngsters and I cannot afford to buy a number of decent sized specimens at ~£40 a pop.
but it was a tennis ball with a catfish wrapped round it! It couldnt even lay flat in the tank, was forever face up or down rocking on its huge belly! Though in all fairness it was in a sump tank with malawis before i 'rescued' it and i dread to think how many it ate because the beastly thing was a good 20cm long not counting whiskers and tail! 
