calm down and keep cool.
Really. That's like the single most important thing. This is not to say you should be oblivious to the gravity of a serious situation, if there is one, but fretting out will never help you. You don't need to prove anything by acting agitated — nor can you, really.
So try not to be too agitated when you read, think and reply.
If anxiety mounts up too high and it's impossible for you to stay calm without knowing what's there, take a look, get the gist of it, still take a walk outside before replying.
Next, there are complaints, and there are complaints. And then there are complaints.
Ordinary 'complaints' are less real than a real, formal complaint.
Simply sending you a revised version of your work is not a complaint. Nor is asking questions and voicing doubts. Or even pointing errors out, or even commenting on their gravity and number. It hardly is a complaint if it doesn't make some sort of statement to the effect of being disappointed and wanting money back (or some other form of redress).
Clients who do want money back — which does not always necessary require a full formal complaint but sometimes happens in informal negotiations — come in two sorts: the one is legal-minded, the other is business-minded.
The legal-minded sort of complaining client seeks a legal sort of remedy, whether justified or not. He relies on a legal sort of right that results from poor performance. It may be based on simple fairness, but it's about right and wrong, moral or ethical codes and so on.
The business-minded sort is essentially revisiting your price negotiations, whether in good or not so good faith. Fairness may be called upon but not really articles and paragraphs. In any case, it's more about business than about moral or ethical principles.
Once you distinguish between the two, it may be a good idea to learn to use a different approach with each type.
The amount of rebate sought by either sort is quite subjective. It can go either way, really, although with time you will notice some patterns. For example the more business type may be after substantial discounts based on lack of subjective satisfaction (as a legalist you'd frown at that sort of thing or even be incensed at it) but not really want to take your money other than not paying you. By contrast, the more legal type could be less whimsical and flimsy about the grounds but actually go after you for damages.
Thus, in practice, although the lawyering sort would need (to at least trump up) a shade of a credible ground to claim any substantial discount, whereas the negotiating sort could claim a 50% discount based off of sheer lack of satisfaction, but the business sort could still let you off the hook after reducing or denying your pay, without coming up with a long list of damages and losses general, special, direct, indirect, circumstantial, incidental, exemplary and totally imaginary.
Either of the two sorts can use two different styles: collaborative or adversarial.
This pretty much means what it says; the labels are self-explanatory.
Be aware, though, that just because someone talks softly doesn't mean he isn't carrying a big stick. On the other hand, just because he's talking tough and throwing his weight around doesn't mean he wants to blast your kneecaps. This is, essentially, the meaning of style. Whether they want to really collaborate with your or be your adversary is quite a different thing.
The adversarial style is likely to resort to unseemly things such as yelling, insults, exaggerated demands with enough padding to go down a notch or five in a settlement, possibly some vague, frivolous or outright fake claims, like grapeshot of which some will hopefully stick. Don't assume it's for real, but don't assume it isn't. Only time will show. So be prepared but don't fret out and don't let them see you're scared (if you are).
Bottom line: stay calm. Even if the situation really is quite serious, you don't need to freak out to prove that you understand the gravity of it. A level head will keep you out of trouble.
The next essential thing to always bear in mind is that:
just because they say it doesn't mean it's true.
Granted, you already know this on an intellectual level, but to really believe it is a whole different matter.
Not only in theory, but in practice that specific client who's complaining about your work specifically, who is complaining right here and now, may very be 'tangibly', provably wrong. So find out.
No matter what you may have been told by Stockholm-syndrome-afflicted translation teachers, speakers, writers, bloggers and all the other assorted gentry, your client's word is not the law.
Your client's word is not the law of the land and especially not the law of grammar and style. Nor can your client be both a party and judge. In short, your client is not the last arbiter. The court is, and not the first court that enters a ruling in your case, either.
So don't associate your client's claims with too much finality. Rather, what the client says is tentative. And it certainly is likely to be warped, one-sided, partly self-serving, partly misinformed (who's the professional at your job?).
Next, and in connection with what we've only just covered:
any huge demands are likely to be phony or desperate or psychotic or just intimidating and softening you up before real talks or any combination thereof.
One million dollar in damages, really? You get the point.
If it's something that the client's QA found, relax and breathe, the damages-generating event most likely hasn't happened and cannot any more.
Unless some sort of one-time opportunity was well-documentedly lost due to your fault, or an absolutely final, non-restorable deadline was not met, with some disastrous consequences, you are most likely not liable for anything more than a reasonable reduction — if they even have a cause for one. It's down to defective performance now (if it really was defective), and you're normally entitled to a go at fixing it before money can be claimed from you.
Aggressive negotiators are likely to try and charge you for the reviser's bill, except to really be able to do that they'd need to have consulted you first, to prove that you were unavailable or would not have been able to fix the problems that they found, and, obviously, that those problems were real. Not to mention that they can't just go and pick the most expensive old fox of a translator to finish a job they assigned to a green rookie to save some bucks.
it probably isn't going to get worse than non-payment if they don't have a surefire case, anyway
Granted, they can declare all sorts of demands, since it costs them nothing to try, but actually going after you with a lawsuit, especially in a foreign country, especially on a different continent, is a whole different thing altogether.
And most people will not sue you for the kind of cash that won't cover the lawyers' bills. Unless perhaps it's personal (so don't make it), but probably not even then. Or unless they have unlimited resources and something to prove — some corporate lawyers do fall into this category.
As for non-payment, it isn't too easy to oppose a valid invoice without solid, concrete proof of bad performance. Even if you don't sue them (for example because the money is too little to bother or because they're in a different country and legal system), which you'd stand a good chance of winning but likely need to incur heavy costs out of your pocket (ironically, most of those could be translation fees), insisting on non-paying a persistent translator is a reputation risk many people would not take without believing in the justice of their cause.
it's not the end of the world if they found some real issues.
Let's not take things out of proportions. Culpably introducing a fateful error in a medical translation or safety manual and causing someone's injury or death is one thing, failing to spot a type or typing in the wrong word in a normal business text or press material or something people read for pleasure likely isn't worth a discount at all, other than perhaps a token couple percent as a show of your goodwill, at your discretion. Which, actually, I would often advise that you grant liberally in order to be fair to your clients and preserve your relationships with them.
don't admit to errors you didn't make.
Whoever thinks client service requires owning up to mistakes and wrongs you know you didn't do needs to... well, never mind, that person is simply dead wrong.
As for real errors, I'd say don't get defensive, rather own up and make up, but I need to make the disclaimer that admitting your liability means admitting your liability so you can't sue me.