with my 550f i got off really easy: it had a nasty pseudo-psychadelic paintjob. other than this eyesore i haven't come across anything weird yet.
several years ago i bought an ss50 (ok it's a moped but a honda). someone thought it was a good idea to bolt on a nasty cheap chinese carb that was far too big, and a throw in a camshaft that only wanted to produce power after the valves would start floating... it was otherwise in really good original state.
the guy i bought it from admitted that he was really good with cleaning it but not so much with the mechanical stuff.
of course, it hardly wanted to start and on the rare occasion it did, it couldn't stay idling. i looked at the guy: "sorry man, i'm really taking a risk here with a non-running engine but am happy to take it off you if you drop the price"
after changing it back to a reasonable setup the little thing has been running like a champ ever since