Level 99 Hacking
From Diablo Wiki
In early 2002 there was an outbreak of Clvl 99 hacking reports, which were ultimately unverified. Plenty of players reported their level 99 characters had been emptied of their gear, and rumors were that it was possible to create a player of the same name on a different realm, or a very similar name on the same realm, and then somehow trick Battle.net into letting you log on with the real level 99, instead of your newly-created similarly-named character.
This was never confirmed, and the reports of this all came in around the time that a great deal of regular account thieving was going on. So it wasn't clear if the level 99s were being hacked, or if people's whole accounts were being hacked and they just noticed it most obviously on their 99, who was wearing their best equipment.
At any rate, the reports died down after a few months, and here's the original report on it, from the warnings archives.
Clvl 99 Hacking
March 9, 2002
This is a controversial topic. Blizzard has recently posted that this isn't a real issue, and that it's just rumors. The forty or fifty people who have mailed us over the last 3-4 weeks to report that their characters were hacked will probably be surprised to hear they are imagining things, though.
In a nutshell, lots of players have reported logging on to find their Clvl 99 character stripped of all good equipment. This differs from the normal account stealing since it's just one character on the account, not all of them, and it's almost always the one very high level character, one that's visible on the ladder. All players reporting this say it's not just that some duped items might have vanished, but that virtually every item they had on, and in their stash and cube, are now gone.
Based on the number of independent confirmations of this we've received, we don't have much doubt that it's a real issue, whether Blizzard believes it is or not. We've not heard any methods to hack like this, but that could just be because the person who knows the trick is keeping it to himself, so he can profit and Bliz can't fix it. Time will tell.
Update: We've gotten zero new reports of this since mid-February, after we were formerly getting several a day and at least a dozen a week. This would seem to have been fixed by Blizzard, though they've never said anything about it publicly. They either found how it was being done and closed the loophole, or they fixed it by accident, while fixing some duping or other realm flooding techniques.