I haven't noticed people being taught to belay improperly. Maybe that's more common in sport crags, or maybe I'm just not paying attention.
One thing I do see all the time though is newer (or sometimes older!) leaders placing active gear "sideways" -- not alligned in the direction of anticipated pull, but perpendicular to that pull. I've heard many of them say, "oh, the cam can always just rotate. And besides, it's a flexible stem!"