To Let Ephialtes Join Or Not
Leonidas, in spite of being a wise commander, doesn't seem to realize how stupid it is to kick Ephialtes to the curb when he's only other guy who knows about the critical weakness in the Spartans' strategic location. Leonidas refuses to allow him to fight with the Spartans because he is physically unable to hold the line, but he could at least have offered to let him fight with the "brawlers" from the other Greek city-states. At the very least he should have preventing him from just wandering off. It might be explained by sheer Spartan arrogance, however, in assuming that Ephialtes could never be a threat to them.
- Or that Leonidas simply (and ultimately foolishly) had a far better opinion of Ephialtes than most give him credit for. He saw that Ephialtes desperately wanted to be a "real" Spartan... since he believed that colluding with the enemy was something no true Spartan would do (again, something he turned out to be wrong about), he simply never considered that Ephialtes might betray them for personal glory.
- It's also noted that in the Deleted Scene, Ephialtes immediately tries to throw himself off a cliff after this rejection. This shows that Leonidas was probably savvy enough to recognise that in battle, the man in front of him was so desperate to prove himself, he would likely end up pulling a Leeroy Jenkins manuever. He even points out that since the whole point of the Phalanx is to protect the guy next to you as much as yourself, so going off half-cocked is simply not behaviour to expect from a true Spartan warrior.
- Not that any of the Spartans actually stick to those rules once the Immortals show up.