The most disastrous errors are associated with extreme cards like Fractal (targeting the wrong creature loses them the game), Miracle (easy to burst past because it triggers use too late, loses them the game) and Shard of Sacrifice (plays it at 12 damage, makes it much easier to overcome than it should be, loses them games).
Some other huge problems are with using PC too early or on the wrong targets; Steal and Deflag being used on pends/pillars before important perms are played has made them lose thousands upon thousands of games, as well as baiting PC with things like Hourglasses.
Incidentally, those are 5 cards.