In 1985, I bought an 1801 Half Dollar for $85. I took it out of the 2 X 2 card board holder, and placed it in a plastic sandwich bag. I then tightly wrapped a rubber band around the bag and the coin. Not sure how long I left it that way, but when I removed the coin, it had a dark streak across it where the rubber band was. I took it back to the dealer to ask if there was anything I could do to fix it. He rubbed the streak with his fingers, and it became lighter. However, you could still see the scar left by the rubber band. Years later when I submitted the coin to PCGS it came back Genuine AG3 Details, Tooled.
Worst mistake: Taking a large cent out of a slab to finish a Type set Dansco and damaging it in the process. I wasn't careful and cut the rim of the copper coin (that was a sick feeling). On slab -> Dansco conversions: I took about 10% of my 7070 coins out of slabs, but none of them were MS or PF graded. And while I get that slabs are appropriate for expensive/rare/nice coins, there's nothing like holding a nice, old XF or AU coin in your hand vs. a plastic case and not worrying about its value all the time. I've also done the whole, "scratch the coin taking it out of a 2"x2" holder", thing once... ONCE...