Someone gave me really harsh critique and pointed out lots of stuff... I meant to make that leg extra long, it's not a mistake!
The only thing harder than giving critique is receiving it. Many people ask for critique but don't REALLY want it. They're only interested in praise, or what is generally termed as 'ass-patting'. Everyone, at one time or another has been irritated that someone was harsh on their work pointed out every single mistake, but you should try to appreciate their point of view and avoid taking anything in critique personally, unless they're attacking you, in which case you should report that behavior.
The first natural reaction people have is to make excuses like "That's just my style, I meant to do that. This is just a practice sprite anyway, so it doesn't matter. I was just messing around, it's not supposed to be perfect." If you find yourself doing this, just back away from the computer for a bit until you can calm down and then take a serious look at your sprite and see if there are some valid points in the critique you've received. It IS entirely possible that you really DID intend some things, or that you really WERE just focusing on coloring and not the anatomy, or what-have-you, and you should express that when you first post the sprite, not just after someone calls you on it.
And be honest with yourself... if you didn't consciously decide to do it when you created the sprite, it's probably a mistake. And that's absolutely fine, because now someone has pointed it out to you so you can fix it.