I am not sure how I can see if certain bugs have been reported, but here are a couple that I have found. I was given the following side quest: "Eliminate Lord Dredmor's 34th regimen, the Mother of Owl, claim Fwnus, the Fleeting Cake and destroy it at the Mellow Shrine! [Level 1]" I have searched every nook and cranny and cannot find where to complete this. I have searched level one about 5 times now. I am positive this is not a case of just missing it. The other bug is regarding double summons. Just save and quit when you have a summon, and you can summon another one right after you reload. Only lets you have two, though.
You don't actually get a message when you destroy the regiment, and only the last creature you kill drops the item. You might have killed them all, the last dude was hiding in a corner, he dropped the quest item and now it's hidden from sight. Though actually you weren't clear now that I look at it, do you mean you can't find the Mellow Shrine?
Then my first paragraph. What it was referring to was a named group of enemies (they usually take the form of a bunch of at-level enemies, led by a uniquely named and uniquely colored version of that enemy with sparkles around it), which you probably wiped out without paying attention. The item would have dropped when you killed the last one, it's probably out of sight behind a wall somewhere.
Hmm... now I'm really confused... went through again and still nothing. If it's hidden, it is hidden well.
Sometimes the enemy does not have sparkles around it, and it is only uniquely colored (like the maroon Diggle I saw on level 1).
I have had the same quest-related issue on level (3 I think) where I am positive the enemies never appeared - indeed, I had already cleared out most of the level and would have noticed if I'd come across a group of enemies. I wasn't paying attention until later in the game, so I wasn't sure if it was possibly a bug caused by i) saving, quitting and loading later, ii) leaving the level and returning, or iii) having two quests on the same level at the same time. This was on my first playthrough, so I put it down to noobiness (noobicity? noobality?) at the time but I still have the save game there, so I might check it out again.