My worst and probably most humiliating loss was this:
Not too long ago I finished a fort while in an alliance with a sloop and brig. The sloop crew joined me in on the fort while the brig stayed nearby because we noticed a galleon was holding position just outside cannon range of the fort.
The enemy began sailing towards us right as the Skellie captain fell. The allied sloop crew then told me to just load everything onto my ship and they'll buy me time. One of them took off in the sloop, the other stayed with me and ferried treasure onto my ship.
My allies fought off the galleon as it tried to break through the line and collect the loot. The enemy turned tails and sailed off after a short skirmish.
The allied sloop crew that was with me, ran off to join his mate. He cannoned himself off the fort towards the ships. About a minute later, they proceeded to attack each other as a message flashed on screen that a ship had left our alliance. I realized what was happening and started to double-time my effort to load what treasure was left.
Once the sloop sank, the brig called out to let me know all is well and that the sloop was planning a mutiny. I checked my map and sure enough. the brig was still marked as an ally, but not the sloop.
As I finished loading the last of the treasure, I asked if they wanted any of the treasure in particular to earn higher gold or XP on those items. They said thanks but it's not a big deal. As I started to raise my anchor, I heard the familiar sound of water splashing nearby. As soon as the anchor was up I dropped sail and ran to my cannon....I was not fast enough. I got taken out by cannon fire from the brig and was promptly sent to the ferry of the damned. While awaiting my resurrection, I saw a message flash on my screen.....the alliance has been disbanded.
Shortly after my arrival on the ferry, one of them showed up here as well. He apparently blew himself up in order to deliver a message.
He said "No hard feeling mate, but why settle for full gold/XP on some loot and half gold/XP on the rest, when you can get full gold/XP on all the loot."
It turned out that the brig and sloop were both going to double cross me by disbanding the alliance and attacking me, but the brig crew fooled the sloop into disbanding first so it would look like they were the only ones.
Ever since that day, I refuse alliance invites when I am solo. I only join when in a crew, and even then, I am at the ready with one hand on the sword, and the other on my gun.