InformationWeek discusses a recent lawsuit that Google is facing over some ambiguity in an ad.
(Photo courtesy of Valleywag)
More specifically, from the image above, you see that there are 2 boxes. If you leave the second one blank, it doesn't imply that the amount is zero. Google implies that you're using the Default CPC bid for the second box if left empty.
Understandably, some forum members are thinking that Google is in the wrong here. It may not warrant a class action suit, but it still warrants something. In general, Google's CPC input boxes have been a problem before -- one forum members says that if you miss a decimal point (e.g. you set your ad to $015 instead of 0.15), you could end up paying a LOT more than you intended to.
Other forum members believe that this is linkbait at its best and that Google AdWords has help documents explaining all of these fields. If you don't read the fine print, you shouldn't hold Google liable that you've lost money. You should also not "test and forget" your ads; if something looks fishy, stop it before you lose money inadvertently.
Forum discussion continues at WebmasterWorld.