The one that stands out for me is buying a BSA Improved Model D in .25 from auction via a friend sight unseen.
I paid around £300-£350 for it but when I saw it, it had been refinished in a gloss black rather than blue and just looked awful. The markings were all clear, suggesting there was no pitting present before a previous owner decided to refinish it. You could also see your reflection in the stock - it must have been varnished.
I replaced this rifle years later with another .25 IMD that had been verbally described as 'mint' by the owner prior to finalising the deal at a fair. When that rifle came out of its bag, it had a shiny gloss grey finish that was no way original but I bought it anyway as I didn't have a .25 at the time. It shot better than my previous .25 but again, I could not live with it and sold it on.
Luckily I now have a much better example!
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.