Continuing Borderlands/Torchlight week, there is an oddity with the scavenger quests (and possibly others), and it does not seem to affect everyone, which implies “bug” to me. It seems that, if you pick up quest objectives in an order other than the one in which the quest guide points you to them, the quest guide breaks. The little diamond on your map points to nothing in particular or perhaps one you already picked up. Luckily, the item(s) will still be in that general area, so you can just look around for a while; it helps to do this at night, when the green lights stand out.
Checking if I was the only one with this problem, I found many defenders of the current implementation. Their theory is that this is intentional behavior, that the quest guide is only supposed to point you in the general area. Some lean heavily on “scavenger hunt,” which is arguable even though scavenging has a meaning prior to (and hence yielding) scavenger hunt. They must believe that the first few quest points are exactly on the right spot as a way to ease you into it, at which point the real difficulty kicks in with no notice.
The sad thing is, I cannot say with surety that they are wrong. (Some of course claim surety that they are right. Dev comment, anyone? [Update: Scott provides, yes, a developer “known bug” quote.]) I have seen far worse “working as intended” statements. Anyone have the classic CSR response quote on the Everquest raid boss that spawned below the world, something like, “This is a challenge!”