I mostly just run ToNG these days - with the occasional mSP, mSVA or FBI thrown in, but for the most part it is just ToNG, ToNG, ToNG. I think I have run into every bug or issue there is in ToNG - some of which have workarounds that not everyone may be aware of.
Anyhow - here is a list, with some thoughts on workarounds.
People are always complaining about lag in ToNG - even those with a very fast connection who have never any lag issues anywhere else. You get very jerky movement, delayed response to keystrokes and other similar problems. Sometimes the issues seem to go away if you just go to the "Change Character" screen and re-select yourself - sometimes a full relog will seem to help, but too frequently nothing will help, and you just have to suffer through this.
Sometimes when you go in to fight Orcus he just doesn't show up. So, you just stand there and look silly for a while - and then suicide to get out and try again.
Sometimes green balls appear after Orcus is dead and the TempHP buff wears off. This is annoying, as it is pretty much a guaranteed insta-kill. There is no workaround as such. just focus on grabbing any loot from the center before the ball reaches you, and then just use your death to get out of the area.
Rock trap bug
Sometimes the rock trap will bug before you clear it away, so you end up stuck and cannot pick up the remaining rocks. There was some speculation that this was timing-related - if two people attempted to pick up or drop a rock at exactly the same time, but this has not been confirmed. Now, there is a workaround, but it borders on being an exploit, so I will not discuss it here, except to say that it requites a particular class using a particular ability and then everyone else just suicides to get through.
Potion puzzle bugs
If one player drinks a potion, while another runs through the room and triggers a trap, the first player may be stuck in a shapeshifted form, with blurry graphics for the rest of the run. There is another issue which I have seen a couple of times where a player may have multiple, overlapping forms - basically a rainbow - green dino, yellow, orange, blue...all at the same time. I have no idea how to reproduce that.
Occasionally the Tomb Dwarves just before the last boss will get aggressive and attack for no good reason whatsoever. I assume this is related to some passive power or specific companion behavior, but I have never been able to track down exactly why it happens.
There was an issue where you would reappear at a distant campfire - even at the very beginning - if you jumped or fell off the end boss platform. This is, however, supposed to be fixed now, I think.
The missing chests
This is probably the most annoying bug of them all, where you don't get the chests after killing Ras Nsi. The general consensus seems to be that you can reduce the chance of this happening by slowing down or just stop attacking him once he goes down to 2-3%, and just let him be swept off. This bug has gotten developer attention, but there is no ETA on a fix.
Did I miss any bugs?