My douchebag comes in the form of the person who, while you are talking about something you really like and would want (say a certain type of dog), they are suddenly the expert on said dog. An expert on why you DON'T want said dog. Because it's a killer, or it's messy, or just because they say so.
The know-it-all-douchebag is one I know well. She's my step-mother. This type also gets a lovely condescending look in their eye. It's my favorite.