(again). Let's face it. Any translator who has been working on a regular basis has made mistakes that were noticed by the
client. Even if you thought your work was flawless, someone might have a different idea about it. Then there are those gaffes that somehow got away that undeniably lowered the quality of a document.
Well, what's done is done. There are two things that I do in these cases (besides lament), and they are: deal with the immediate situation with the client and set up a strategy so that the same mistake will hopefully not happen ever again.
Let's start with dealing with the client. They may either nicely or annoyedly contact you and tell you that on page such and such, there is missing text or a mistranslation or terminology issues, or, gulp, all of the above. The first thing I do is take a deep breath and check my pride and then refer to the pages that the client remarked on. Sometimes the client uses different terminology than you did, and that's simple enough to clear up, but objective mistakes are not. After getting the picture, it is time to respond to the client.
Tell them that you are sorry and that they are either partially or entirely right. If there is a terminology discrepancy and they did not send you a glossary, you could tell them that you will fix the issue but that you would appreciate any glossaries and references in future jobs. For the rest of the errors, offer to correct them free of charge, ASAP.
Now, for some reason, if the mistakes are so numerous it's embarrassing or the client requests one, you could offer a partial discount to offset the inconvenience. You can't expect the client to pay full price for something that doesn't merit it. You might possibly save the client this way as well depending on your prior relationship and how long you have been serving them.
If the quality is abominable because you didn't do a spellcheck or you were asleep while translating that particular document, just cancel the PO and call it quits. It's a terrible thing, but it will be a learning experience.
So how can you prevent these particular mistakes from encroaching on your screen again? I would wait a few days before doing anything with the text. I would wait until I had a quiet moment and then make notes in the same folder as the document in question. What went wrong? What did you do wrong? How will you prevent it from ever happening again? The answer usually lies in the pre and post translation processes. Before starting, did you check the style guide and glossary? After you finished the translation, did you proofread it thoroughly and save the file?