Taking a 1820 Bust Quarter out of a stapled 2X2.....put a BIG SCRATCH on the obverse. Happened 9-10 years ago and has been sold and it still bugs me today...
I had purchased the nicest 1670 Bristol halfpenny token I had ever seen on eBay, it was mailed from Great Britain but unfortunately came on the very day that I had gotten news regarding a family member. I know I had received the token, looked at it and probably put it back in the envelope. It disappeared in my haste to travel out of town, and my best guess is I figured the token was not in the envelope and I tossed it in the trash.
Worst coin accident was doing a crack out 4 years ago. I had the pliers out, and was cracking a PCGS OGH with what I thought to be an under graded Morgan in it. I successfully cracked the slab, but it splintered, the coin rolled out of it, hit the table and bounced on the floor--causing the coin to be a permanent no-grade with multiple scratches and rim nicks--instant MS 64 to crap. Even worse was a fragment of the slab lodged in my eye, and I had to go to the ophthalmologist, and have him remove it with the microscope--corneal abrasion.
My worst coin accident was 2 hours after my first coin show. I had just returned from a great time in Vienna Virginia and was opening my door. (I live on the 2nd floor). I somehow managed to drop my coin box and low and behold a vg 1806 Draped bust half bounces out, takes a bounce on the wood and falls between the cracks (20 foot fall). Thank god it landed in a pile of leaves. I bolted down the stares thinking that there would be a huge scratch. It actually ended up scratch free and I ended up selling it to a CT member.
It was a common date--an 1883o, but a GORGEOUS example of the nicest that Nawlins could make--super nice strike, dripping with luster, and a proof like reverse. It was a sure 65 or 66, as it looked like the grader was hungry or had to go to the bathroom when he graded this one. Lesson and word to the wise---wear safety glasses, and do the crack out on a sofa or the bed.