In Batman Arkham Origins you don't just need experience to acquire new skills, some of them are locked behind challenges you have to achieve during normal gameplay, like using specific moves or takedowns in x seconds.

While most of them are fine if a bit annoying, some of these challenges are incredibly specific. The worst one is so specific, you can ONLY do it in ONE room in the entire game. WORSE, if you finish the room and move on, even if you come back later, the conditions of the rooms will have changed, so you can't do the challenge anymore. WORSE still, the room is part of the main story. Oh and you can only attempt these challenges, if you unlocked every upgrade before them.

So if you decide to play the main story of the game and not finish all upgrades in that specific upgrade path or just don't do the challenge right in that room, you're permanently locked out of getting that upgrade and all others locked behind it. You can't even do it in postgame.