There were a bunch of times when some less experienced wizards (read teenagers) were able to temporarily beat much more experienced wizards by an element of surprise. On top of that, wizards were pretty much powerless, if a wand was knocked out of their hands.
This raises a question: Why don't wizards (especially those who have to participate in some battles) wear some magical protective items (to protect from simpler attacks, to get back their wands and so on)?