Boxing

[BitVyper]

1. Failing to take into account that at some point, you're going to have to fight dirty and
2. Failing to take into account that a real opponent probably WILL fight dirty.

I can't speak for everything, but I know from experience that Shinkage ryu kenjutsu deals specifically with number 2. Especially when it comes to tanto and fan stuff. Granted, a lot of that isn't really applicable anymore, but it is there. What little I know of some softer styles seems to address it as well, probably because there's more opportunity for nastier things when you're grappling.

Another thing to consider is that for stuff that was actually being used, students were probably picking up that sort of thing from experience since they weren't necessarily spending twenty years training or fighting in the ring without ever taking it into battle. Probably part of the reason it's not built into a lot of systems is because in times of war, you might be learning it for real before you'd even built a strong foundation, and in times of peace it's easier to have the idea that everyone will fight by the book.

That's just speculation though.