lol
their worst enemy
is themselves 
Agreed.
When D&D 4th edition came out I had volunteered to DM for the demonstration games at my local store. No-one was really familiar with the system (it was all of 5 hours old, give us a break!) but we'd finished the prepackaged adventure and my players were hungry for more so I started to invent my own dungeon pretty much on the fly. Nothing complex, just a couple of fights and traps with a bunch of loot in between. I managed a TPK with a trap.
Sounds like either unfamiliarity with the system or bad GMing, right? A trap that can take down the entire party is generally bad form for a low-level dungeon after all. Let me give you the whole story.
The trap in question was a spiked pit. Ten foot by ten foot by ten foot, it did something like 2d6+3 damage in a game where the lowest HP character had 28 hp. it was crude and required a dc 12 check to spot, which was below most of the characters passive perception. beyond it was a dead end. So the characters spot the trap and the dead end and decide that there must be a secret door on the other side (there was), so they decide to cross it. the mage goes first and fails his balance check to skirt the edge of the pit, falling in and taking a bit of damage. he throws his rope up for a friend to grab an help him up.
his 'friend' steals the rope and gets ready to leave him.
unsurprisingly the wizard is unimpressed and uses one of his encounter powers to pull the other guy into the pit, then jumps on him and uses him as a springboard to jump up and grab the edge of the pit. the other guy grabs his ankle and pulls him back down.
now the monsters in the next room have noticed the hoo-har and poke their heads out. they see the adventurers antics, look at each other, shake their heads, and head back into the room to get the cauldron of boiling water they'd been planning to use for stew. the adventurers were so busy fighting each other they didn't realize there were orcs around until a few hundred gallons of boiling water got tipped on their heads.
It was on this day that my faith in players was irreparably damaged.
So yes, players are their own worst enemies.