Pre-patch heavy crossbows. The oldest of veterans know what I'm talking about.
Getting poisoned for the first time, I think from a snake, and running to the WBB. One twit jeered, "Oooh are you about to die?" Another person cured me.
Spending my last 900 gold to surprise a friend, who'd just been PKd again, with an AR30 plate suit.
Everyone wearing full plate, and often a robe so PKs wouldn't know. Mages wore magical wizard hats. Gaining 5 int was worth losing 5 str and 5 dex.
People believing white horses were faster, that eating food helped your casting odds, and that moving after fizzling helped your odds.
Thinking that when I called for guards, I had to name the person (usually a thief at WBB).
Stolen items remaining on thieves' bodies, so they'd steal and run across the screen to an accomplice, who'd loot the item.
Every noob thief complaining on Stratics that guards returning items was ruining "roleplay."
Every jerk excusing outright sexual harassment and racist slurs as "Im just rping."
Thieves stealing outside town and remaining blue, then giving a murder count (no thieves guild at the time!).
Becoming "Dastardly" under the old notoriety system if you accidentally attacked blue friends a few too many times.
My friend who introduced me to UO saying, "I'm a Dread Lord, but only because I accidentally attack friends a whole lot of times."
Becoming a Great Lord by killing a single rat every 15 minutes at the Jhelom docks.
PKs regularly at the crossroads, and outside the Brit and Trinsic moongates.
People shouting at the WBB that there were PKs at a certain place. Often the Great Lords, if they drove away any Dread Lords and saw other blues returning, would return on their own Dread Lords to do their own PKing.
And for the reason above, it's a myth that Great Lord/Dread Lord wars at the Chaos Shrine had anything to do with good guys and bad guys.
"vendor buy bank guards" that later became "vendor buy bank guards recsu recdu" with the advent of T2A.
Archer suits, after the early 1999 PvP revamp introduced dex penalties.
Early characters that got corrupted and were simply wiped. No backups offered. Fortunately it didn't affect me, but my guildmaster lost a master mage. That was a big deal.
His silver heavy crossbow of power, worth 25K (a small fortune). I thought he was a great guy until he almost laughingly told me how he got it: he scammed it at the Brit forge, pretending to be a smith.
Some characters were bugged in total stats. I had one that peaked at 228, my GM had one at 235. Some couldn't get to 225.
Shopping in different cities because some NPCs were cheaper. Moonglow had black pearl and blood moss for 5 gold each, while everywhere else was 6 gold.
Carving bows for four nights to get the 40K for my first house.
The land eventually becoming full of houses, every coastline full of small houses, because they were unlimited per account.
Learning that my new neighbor was a cheater when my GM swordsman -- maybe one of 50 at the time on the whole shard -- could hit his unarmored noob only once every 10 swings. He swung a halberd as fast as a dagger and connected every time. He didn't do much damage, but neither of us could heal, so he won.
Losing that house because, whoops, I had the key on me.
My friend confronting this guild, who dared him to page for all the good it would do.
Learning that one of the most respected RP leaders on my shard, in fact a Seer on another shard, was secretly one of the most notorious PKs. So he'd know exactly where and how to grief his own events.
The most infamous cheat program of UO's early days, and the bane of my play: it gave PKs bows that fired like Uzis, instant recall (quick escape for PKs, and this was years before the aggressor flag), and "ghostwalk" long before stealth was introduced. We once chased a guy through Shame after he killed our friend. My high tracking saw where he was going, but when it indicated we passed him, we realized he was completely invisible.
The same program allowing snooping from across the screen, and stealing multiple items at once.
Zig-zagging so a mage couldn't target me, only to find he was cheating with Last Target (before UOA was legal, and before you could target player bars).
GM Darwin.
Netting a whole 8K an hour from making fancy shirts, when unattended macroing was legal.
I was likely not the first to discover the fruit basket on my shard, but I was evidently the first to get it regularly. For that first month until the secret got out, it sold every day for 25K -- great money at the time.
Server down at noon.
Server downs at irregular times, two or three times a week, with time warps.
The really bad time warp (1999?) and server reverts that erased three days of play. It started with one night of horrible lag. Several of us were sitting in a player-run tavern outside town, but there was no danger since would-be PKs were lagged out too.
By late 1999, server down could take 90 minutes. Without house decay, before secure housing storage, the tiniest of houses was loaded with junk.
The dupers who realized what overloaded houses meant.
The one guild who found a way to crash entire shards.
All of the MDK guild getting banned.
Fighting balrons in Hythloth 3 using teleport pads.
Barding without line-of-sight, and owning the terathan keep when T2A first opened.
Not being able to move the Trammel was scheduled to open, because of sheer lag just about everywhere.
And most importantly: asking my best in-game friend to tell me once he spotted my rarely seen game crush, who ended up becoming my best IRL friend.