When I'm at CompUSA, et. al, waiting for something when the person in front of me is getting advice from the salesperson, it takes a whole lot of will power for me not to say, "No, he's wrong. He's what ya wanna do...."
Like when that ZipCD was on sale last weekend I heard some dork there asking if the ZipCD was any good, obviously not knowing about it being a rebadged Plextor. The sales person told him, "It's alright, but I like the Yamaha 16x burner, it's the best on the market." I was about to say, "Maybe the fastest, but not the best. That ZipCD has the burnproof which prevents coasters and that Yamaha doesn't. Saving that 2 minutes really isn't worth it when you can end up making a coaster and having to do the whole thing again. No to mention the reputation of Plextor.. etc.." But they were sold out of the ZipCD anyways so it didn't really matter.
When I'm with less 'enlightened' friends, I'll usually really tone down my knowledge unless they start quoting stupid sh!t to me or talking to someone else and giving bad advice.