What is the biggest mistake you've made grading a coin?

RKKay

Whether you mean moneywise or point differential in grading a raw coin which was ultimately slabbed (or BB'd for that matter).

barberlover

I'd have to say evey single mint state flying eagle cent i've ever seen [they all look like rub, slider unc's to me]

Gilbert

Voicing it.

MrEureka

It was 20 years ago - I wouldn't make the mistake today - but I unknowingly bought an incredibly lightly whizzed and gorgeous 1909 $5 for about 5K. At the time it was something like 66-67 money. I immediately resold it to a reasonably sharp big dealer for a good profit. He brought it back to me half an hour later after someone told him it was whizzed. I refunded his money and then went to try to return it to the PNG dealer from whom I had bought the coin. He refused, but I forced it down his throat, saying I'd stop payment on my check. I don't know if I'd handle it the same way today, but that's another story...

TheNumish

Made too many mistakes to remember but last month I bought a toned 1920 half for $800. Thought the coin was a lock 64 shot 65. Sent the coin to NCS for restoration and there was a big planchett flaw I didn't catch and NGC wouldn't grade the coin. I figure it's worth about $200. That's my biggest mistake this year. Can't complain too much because it was my own fault. Started thinking I'm smarter than I am and this brought me back to earth.