On level 3 one of the quests was to defeat a monster, then use the artifact it was carrying on the monolith. I found the beastie and dispensed it with a couple of fireballs. However I found out that it was on a 'island' that only flying creatures can reach. On one side were walls or green liquid. On the other three sides were channels of green liquid. Since my character can't fly, I had to remove the quest. Unfortunately I didn't take a screenshot of the situation. The next time it appears I will.
That happened to me too... Someone on IRC mentioned that you can use concussion bombs to move items around, so if it's the item that's stuck on the island, you can find some of those and potentially get the item that way. (If it's the Monolith stuck on the island, that's more problematic, of course.) Also, if you happened to choose the dodging skill tree (I forget the name at the moment), the skill "Knightly Leap" in there will also let you jump over gaps like that.
Strangely enough, I did have the dodging skill tree, but not at a level where I could use that skill. Didn't have any concussion grenades either.
Well, grenades can be purchased at the handheld weapons vending machines, of course. And you can always go back later on to finish the quest once you DO have the Knightly Leap skill; I didn't get it until a few levels after I had encountered the problem. Of course, depening on the angle you're able to throw them from, it's possible that the grenades might not do the trick either. I'm not entirely sure of the mechanics of those.