Overextending. A lot of people do it, when it's not needed. One of the things a good duelist must be able to see, is when to stop. We call it conservative play. for example, when you have Torrential and 2 Bottomless on hand, you'd set 1 bottomless and 1 torren, not all 3. if you're hit by for example Heavy storm, it would make you lose more cards then you'd want. most ppl know this, and do it this way. however, as they do care for the spell and traps, they forget it with monsters. for example, that's how many people would drop a BLS when the opp has no monsters, when for the same thing you could use a beatstick, for example Stratos or Doomcaliber Knight. A lot of people leave BLS unprotected, and just use it to do damage, which in the end, makes them lose BLS, and most of the time also the game. Try to think about how worth it is to use a card.
Personal example, a war game: one of my team mates had a war game, and asked me for help on strategy. he had 2 monsters on the field, 1600 and 2300, and his opponent had dropped Gorz on the 2300 attack. so, he asked me, should I use Dark Hole? I simply said, no. your monsters will be destroyed anyway by him, let him do the work, and after that, he'll most likely set or summon a monster as well. and exactly that he did, next turn dark hole, and made a +2, instead of a -1. so as you can see, conservative play is important, it can save games.
