I have a few:
*Fads like continuous-hopping by novice brewers just because Dogfish does it.
*Adding fruit, oak, bourbon, or spices if you've brewed less than 5 batches, and sometimes more. This is also a fad.
*New brewers asking for advice to combat a particular issue, and then not heeding said good advice and just rolling with it or making it worse, never learning or growing as a brewer.
*New brewers who think they're connoisseurs in the first 2-3 months of brewing. They subsequently travel to Belgium, buy a few rhizomes, subscribe to 3 beer magazines, visit Oktoberfest, trade for beer on a weekly basis, etc. all in the same 2-3 months.
*People who use the word "Balance" to describe IPAs/DIPAs.
*The entire extract kit bunch who's afraid to move on to building their own recipes, or at least learning about it. I agree that kit instructions are very basic/subpar.
*People who pre-freeze their beer glasses... and the beers themselves (to a point).
*Partial Volume Boils!! I hate them. I'm using the word hate here. I'd rather have 3 gallons of an excellent beer, than 6 gallons of a mediocre beer any day of the week. And yes, before you ask, I whole-heartedely believe that topping off with 2-3 gallons of plain water affects the quality of your beer in a bad way.
*People who use hopped-extract or buy a $29 Mr. Beer kit expecting the world.
*Homebrewers who blindly trust advice, even from the pro's... Like a 1999 Palmer quote on such an ever-changing and highly subjective subject like Dryhopping in 2012.
*Insisting that extract or all-grain is the only way to go without ever trying your hand at partial-mashing.
*Homebrewers who let their wort come to room temperature over the course of 24 hours instead of using an ice-bath or wort chiller or something.
*Homebrewers who stock up on multiple pounds of Citra, Simcoe, and Amarillo... and then waste them for a few years in a loose ziploc bag. A friend of mine does this. I called him a bastard for contributing to the higher demand and taking those hops for granted