At some point in your game, the story for increasing drama may be served better by severely hurting a PC. Hitting a true down moment in a story can come very naturally after a hero is crippled in some way. The warrior loses their eyes, the thief a hand, the wizard is struck lame. They can lead to large moments and give the characters a huge chance to grow, over coming a huge obstacle. Or at least finding a way around it, as they find a way to get themselves healed.
The problem with just doing this is, unless your group is very specifically set up, this can ruin the persons character. Taking the fun out of the game and essentially well, ruining everything. Some people want this level of challenge, others well, quite simply, don't. Especially when they don't know when or if there is a way out of or around it coming up. Arbitrarily crippling their character can simply be a blow, not just to the fun, but also to their trust in you as a GM. However, these moments can still be worth it. So what do you do?
Quite simply, talk to your players before you do it. Don't tell them everything obviously, you want some surprise, but make sure they're ok with it. Ask them, "Hey, you be cool if at some point in the story you get blinded? Then we can work through you becoming a blind swordsman, or getting your sight back". If they say yes, then great, you can do it. Maybe not immediately, but you can do it. If they say no, then you know they're not cool with it.
If you have a level of trust with your PCs and think you can just do it...you can risk it, but do you really want to risk that relationship with a possible violation? Even if you simply mention in the beginning of the game that those things may happen, get permission ahead of time for injuring or maiming before you even get going. It can solve a lot of problems, so you should just go ahead and do it really.