I didn't see this before 3E.  What I'm seeing is that sometimes activatable items do nothing.  This is different from my previous report, where the item cooldown started even though nothing happened.  This time, all that happens is the "click" sound that you get when you trigger the item. The cooldown does NOT start, and if I press the item's key one or two more times, it usually fires correctly.

Items that I've seen this with include Heart of Life, Wand of Speed, and Staff of Renewal.  You'll notice that those are all self-targetting instants: I don't think I've ever had this happen with an item that required a target, but I'm not 100% sure.  Also, I've never had an item fail when I used the mouse to select it (but this may be irrelevant, since I hardly ever click on items in the UI).  As far as frequency goes, I think it happens at least once in most games, but not more than a few times per game.