I never came across many, just a few pathing bugs. I remember being in Lower Guk in the room with the frog that drops the black mace, I think Cavalier or something. There was a fire pot in the corner of the room and if you stood on it the mobs would ignore you. We had been camping there in a full group for hours and I never saw anyone use the fire pot, but a GM showed up and just stood in the middle of the room. There were people coming through from other rooms and my group all stood looking at the GM half in awe, and half confused. The entire group ended up wiping because respawns were happening and nobody was fighting, they were busy looking at the GM. I was a bard so I couldn't do anything, no invis undead, no gate, no feign, etc, so I backed up and stood on the pot because I figured the GM caused this wipe, the least he can do is let me not die. But he said GET OFF THERE I WONT ASK AGAIN! So I got off and got insta ganked by 10 frogs. I never understood that.
Another one I saw was in Sol B, there was a spot where you could use levitate and float over some lava and the mobs would stop pathing to you. So Necros would use DMF, sit in that spot and stack all their dots on Lava Duct Crawlers and stuff and watch them die without needing to struggle to keep them rooted. They patched it pretty soon though.
One other thing wasn't really a bug but someone figured out how you could start a new character and then just sell their starting rusty sword and maybe the food too and get a few silvers. So someone setup a serious manufacturing process of creating, sell, creating, sell, and made a bunch of money. After that they patched in all starter gear to be non sellable. The old game was fun. I remember when duels would announce the winner to the entire server, so during buggy periods when the game was new I used to see stuff like, "COMMONSENSE HAS DEFEATED VERANTSUPPORT IN A DUEL TO THE DEATH!"