The largest loss for me was before AOS, I believe around the Publish 16 era, on the Baja shard.
I had really nice (and expensive) Large marble patio right outside of, I believe Trinsic, on the beach/sandy area.
A known (unknown to me) scammer approached me and offered me 20 million for it (when gold was a fair bit rarer) and I accepted.
There was some kind of exploit at the time, which was fixed a week later in a patch whereby they could use a program to manipulate the trade window when trading a house.
So we went to the marble, I initiated the transfer, but then the window closed, he asked me to take ownership of a small tower about 3 screens away, which I did, but when I clicked the accept button, I took ownership of the small tower and he got ownership of my large marble.
One of the community reps at the time (Canyon) told me it was basically an exploit where the scammer could keep open a cancelled transaction window on their end, then once they opened a new transaction window (whether it was an item, pet or house), accepting the open transactoin, would accept both.
This wasn't supposed to happen, of course, as you could only transfer houses within 2-3? tiles of the house sign and the house deed in the window was for the small tower. I should've just backed out when he started making it complicated, but didn't.
So once I realized the house had been stolen, I paged the GM's and said an exploit had taken place, I was pulled into Jail, as was the exploiter, after explaining what happened and that I had taken screenshots, I was sent back to my original spot and had a message saying the GM couldn't see any evidence of an exploit and they couldn't help.
Long story short I posted the story including the screenshots on the old MyUO boards and a Lead GM got involved (Lead GM Maleki I think he was called).
He seized the house, placed signs on it saying for the owner to page into the GM system, I had a hidden character on my cousin's account waiting around, the scammer logged in and his friend arrived and they had planned to deed the house and steal the signs.
To cut an already long story short, the GM removed the house, deleted the signs, then a passerby managed to place a house before the GM could, so the GM made him deed that house, then a new marble was placed and handed to me.