Being able to summon multiple mage pets at once. Like, ALL the pets. Only lasted for about 24 hours before an emergency patch IIRC.
It was live for longer than that, but not by much. It involved having a pet, summoning an eye of zomm. I don't remember if you killed the eye, or just summoned another pet. Necros could do it as well if they had an item that let them cast eye of zomm.
There was a place near the Shadowhaven entrance to Paludal Caverns. You could get into and mobs would just freeze in their spot until you damaged them enough to put them in a death run. Was at the start of the narrow path from recondite camp 1 to camp 2.
There was a bug in dueling. I don't remember it exactly, but my friend playing a dark elf cleric found it out by accident dueling an sk. It involved a combination of effects. Like rooting, blinding, fearing, snaring, and stunning them in a certain order. They eventually figured out the exact sequence and he could duplicate it. What it did was remove the other player from the duel, but not you. So they couldn't attack you because you weren't a valid target (just like trying to nuke or attack player outside of a duel), but he could still engage them. I don't know if they ever fixed it.
I remember the camp into a wall repeatedly would eventually send you to the evac spot in the zone. Was the easy way across Sirens Grotto back in the day.
I remember being able to disarm magical weapons and them fall onto the ground. We had a monk farm the 2 hander from the froglok king doing that. Disarm would work, the sword dropped onto the ground, and the king would just spawn a new sword.
Not a huge thing, but I found out when they gave everyone a free heroic character. You can create the character, and sacrifice it all the way down, delete the character, and remake it for unlimited essence emeralds. I also found out that divine intervention can proc and save a sacrifice and still generate an essence emerald. Not a huge need for them, but was a nice little perk.
Was nerfed fast. The first monk to get to that part of epic in Kunark. You hand in a note to an npc, get a note back to take somewhere else. Before it was fixed it was the same note, and you could just hand it in again for another nice bump of xp. Guy leveled himself to 60 that way.