When I was still F2P, I was heading down to the Moss Giants in the Varrock Sewers. I happened to notice someone with a Santa Hat, so I said "Nice hat

" to which he replied "I'm quitting, do you want it?" I traded him, and he ended up logging out...Less than 20 seconds later, he comes back, and I trade him again and recieve the hat.
I knew it was valuable, but I wasn't so familiar with the forums or fansite forums for reference prices (at this point, Santa hats were roughly 200k). As I was cutting Yew trees with the hat on, someone came up to me and wanted to buy the hat. I took some advice from a friend (and this was the messup) and ended up getting full adamant (t) plus a Rune Longsword. Turns out Adamant (t) isn't exactly the easiest thing to sell, and I had it sitting in my bank depreciating in value while the lucky guy who I traded enjoyed the appreciation of the Santa hat's value. I didn't end up selling the set of armor till I was P2P a few months later, at which point I had theoretically lost around 500k+.
EDIT - Just remembered...I sold my Saradomin Godsword before going to China for 450-ish Crystal Seeds. At the time, the GE database had quite a nice price, and it looked to be a decent trade. Nothing but nice high prices... Of course, I couldn't sell the seeds until I lost roughly 20M while they sat in my bank. D'OH!