I've been playing for about six months and lost plenty of ships to mistakes. But since they're all under about 20M ISK in value, it hadn't occurred to me to keep track of them. But I just made an 81M ISK mistake while buying stuff for my Planetary Interaction activities, and I thought I should start keeping track.
So, note to self; don't buy expensive stuff with an impatient, loud toddler in the room. Instead of setting up a buy order for 3000 items at 47000 each, I set a price of 74000 (thanks, numeric keypad!) and hit enter. This was done while negotiating timely banana delivery to said toddler. I didn't even notice what I'd done until I didn't see my order in the buy list. "What a coincidence," I thought, "someone put up a sell order for that exact amount at the same time I tried to buy. Huh."
I checked and rechecked the transaction, and got that ol' sinking feeling when I realized I'd massively overpaid. Lesson learned, I hope, and if things go smoothly I should recoup that money with just a couple of days profit from my PI operation.
What are your single most expensive mistakes in Eve?