Your biggest mistake in D2

I've been playing since the game was first released decades back. I've made MANY mistakes, but the dumbest were dropping loot on the Ground to put on other characters by exiting/entering and tempting the lockout gods of early Bnet. I can't tell you how much I've lost doing this, but I did it many times before I finally realized it was a mistake.

biggest mistake , I sell end game torch to merchant under vendor gamble screen....

I just recently (a couple of weeks ago) sold my only perfect Annihilus during gamble. 20/20/8
Many mistakes have been done back in the days too.
But remember guys, life goes on. Don't get mad, get even

I purposely made PvP Zeal paladin with Principle body (100% chance, wow!) and discovered that Holy Bolt only damages undead and is useless in PvP.

Sold my War Travs on my Switch by accident to an NPC - the Iteminventary was full - my Boots was gone

War Traveler

Lost my lvl 95 summon necro with top tier stuff because of offscreen teleporting.
Lost 4 or 5 Beast iron golems.
Accidentally left 35 life summon skiller on the Ground when I was busy managing my inventory and forgot to pick it up

Put skill points into every skill "just in case they might be needed". Long before respec (1.13). Needless to say I haven't even reached Hell...

It would be a long time ago and can't remember what game version it was, but what I can remember is:

1. I usually create game and played for a while (from previous experience that will keep the game alive for a while even when no one were in the game), then drop items on the Ground to transfer between characters. There were times that when I dropped good gear on the Ground and exit, then back with another character, the game no longer exist...

2. Sorceress died in a very dangerous place, then I tried to wear secondary gear to come and attempt to clear the area but died again and again. Finally decide that is too hard and save/exit. When relogging in only to found my main gear is gone (only secondary gear), tried several game reopening but no miracle.