new one: having fun with RoL/hope and a fallen druid. you've got a 13-hope, and then the AI plays a thunderstorm. so what, you're thinking? then the ******* plays 6 dragons in one go... AND snipes your druid, adding insult to injury. always in that order.
another: mutations. better have a lot of fodder, because you are garanteed to have at least 2 immaterial 0/2 critters "improved" at the most crucial time.
trying out aether in bronze league? bad timing, it's a charger spam! better luck next time, when your lobotomizer isn't the last card in your deck!
trying out your new uber-deck in the trainer? 20-0 record? spend all your money on it, and you'll have a 0-20 record on your account.
another one i've seen on the old forums was a guy who built a rainbow that had one purify in it. never needed it, and won consistently. took it out for something else, lost 5 in a row. puts it back, wins, etc... lucky card? more like scumbag...