10 year olds.
Parents need to be properly educated that "Game Experience may change during Online play" = NOT FOR CHILDREN! You are going to meet rapists, arseholes, trolls, and people who realise that your parents ARE that stupid to try to raise you via the internet and make you pay for it.
People who ignore the objective.
The scene is TF2 and you are playing Control Point mode for pretty much any map. The enemies have the point and they are holding it well. Everyone goes into "Kill them all so we can catch the point safely mode", and thats Ok. So your team chooses Snipers and spies only for thinning-out-the-enemy purposes. But Spies are flimsy and they have the point, so Snipers can't find enemies on the point, because they are busy flanking Snipers. Then there's 10 seconds left. The team either A) brings out some real classes and owns the enemies and loses because they forgot that this is CP and kills mean less than nothing, or B) They're are all dead and can't react quick enough because of flinging MOAR SPIES in hope that it may work.
Bad choices for the team.
Again, TF2 (it's my game, so get used to it). So the enemy has set up a pretty good offense/defense. You can either respond by outclassing them skill-wise in every respect, with every member on the team being a pro. OR you could make a cohesive team and be more powerful than the sum of the parts. So because not EVERYONE plays in a clan, or learns every weapon from the bullet-spread up, you need a cohesive class structure. You need enough medics to go around, you need explosives to focus on sentries or their medics, and you need flanking classes to confuse the enemy. And lo and behold, out come the best flankers of all, SPIES. 4 of them. So now you beg your team "4 Spies ain't working, we need some Soldiers, or Demos if we got them". Does everyone catch on? NO! The try to backstab PROPERLY now, try different spy tactics, and of bloody course, try to show those shit spies how REAL spies should work by switching FROM Soldier TO Spy.
Squish.
