It's probably a cliché that failures can be turned into opportunities, but one thing which I remember from my past as a gamer (I never managed to enter the really competitive ranks) is that in order to become good you need to realise that you're bad. You just can't progress thinking you're good and you deserve to win. Being more skilled than your opponent doesn't always prevent losing anyway, even the majority of your one-on-ones.
Anyway, I'm rambling, so let me move on to two concrete examples. I failed at two things do.
Failure: I didn't very clearly communicate my lack of availability to one of my core agencies. I sort of presumed they'd just give it to someone else. They did, but only after trying to get in touch with me when I wasn't there, trying to recuperate after a sleepless night. Bad style on my part!
Consequence: I ended up working on it anyway, as the translator who eventually got it delivered a bad job, which I ended up correcting. Correcting it was more like translating from scratch because there was no time to waste analysing. I did almost exactly the same amount of work, probably did it slightly worse on account of the rush, got paid less than a half, the agency probably spent more money, and the client got the text later than if I'd been the first and last translator. Ugh! What a loss-loss for everybody (I doubt the first translator's going to see 100% of the pay.)
Opportunity for me: Learn to be less hesitant about jobs that I might end up working on anyway. If not accepting them (and decide faster!), at least communicate ASAP. Bonus: Learn to identify strategic stuff better. Plus, it was nice to feel appreciated, and I practiced doing some things I normally don't do, since there was no time to be excessively faithful to the original (shifted adjectives between sentences, replaced standard English with Polish idioms etc.).
Opportunity for the agency: Yes, a window opportunity for the agency does exist in this kind of situation. Say there was a problem, a bad job causing delay, which is bound to happen from time to time with such short turnarounds, and a more experienced translator (or one with better writing skills or whatever) was provided at no additional charge to fix the article. According to some market researchers, a client whose problem with the service was solved expertly and efficiently will show more trust in the future than even a client who's never experienced any downsides yet. Plus, once again learn that they have a good translator on the roster who can do some nice things.
Number two (and simpler):
Failure: I wasn't even awake when somewhat foreseeable client inquiry came in the morning.
Consequence: It took me 10 hours to answer what the total fee would be for a nice, straightforward job from a direct client. And a client from the Middle East, and I like those guys a lot because they're so polite, unlike many westerners these days. There's a chance that, by now, the prospective client has accepted a quote from someone else.
Opportunity: (Re-)learn to check mail before going off the radar for a couple of hours, and make time for it. Bonus: Get into a firm habit of really fast responding. Clients will be happier, and there'll be more efficiency, as in more prospective clients turned into actual clients.
So it's only really a failure if you don't learn. It may take a couple more failures for the lesson to sink really hard (fewer if you're more disciplined), but it will help if you take your failure as a lesson. Sometimes lessons with a book and teacher can even cost you more and be less efficient than the lessons you learn from life. So just cut your losses by sitting down for a while and thinking what you could have done better, but don't feel too bad about it. Learning is a bright side.