I had loot drop into walls a couple times in act 1, but after that the terrain is more open and the chances of this happening are much lower.
You may be able to drag and drop an item into your inventory, or onto a character's portrait, even if your lead character can not move close enough to grab something when you click on it.
There is a way to get a summoning doll through walls, etc, which may allow you to grab loot (
armory trick in act 1). In acts 2 and 3 you have teleporter stones that may allow you to reach otherwise inaccessible locations, as well.
Any stuck quest items may be replaceable:
Inventory files.