Found an error in my thesis
It is normal that a thesis contains mistakes and wrong conclusions. If you did not yet do your defense, it is not a problem. The examiners may find and mentione it. Otherwise, you do it by yourself the day of your presentation, by indicating that honest mistake. You can then correct it in the final version of your thesis.
If your thesis is already in the library and you got evaluated for it, try to talk to your adviser and he will suggest either to ignore it or to add a correction on the website or as an extra document in the library.
Talk to your advisor!
This question could have very different answers, depending on lots of factors:
- How significant is the error? Just in a minor statement, or does it invalidate the main results of your thesis?
- What stage of the process are you at? Have you already defended your thesis? received your degree? etc
- What was your thesis contribution? Significant original results that other people may wish to use? or mainly expository/survey work?
- What are your institution’s culture and requirements around masters theses?
- What is your field’s culture on how to treat errors in published work?
Your advisor is the person who can best judge the answer to these, and should be supportive for you.
That said, as a very rough general answer: in most situations, this should not hurt you badly. If the mistake is minor, or if it is major but the thesis has already been defended/accepted, then probably all you need to do is put the correction on the record somehow (in my field, listing an erratum on your webpage would be reasonable). If the mistake affects the main results of your thesis and you have not defended yet, then it’s more likely you should revise the thesis and postpone the defence. But, again: this all depends on many situation-specific details, so the most important thing is to talk to your advisor about it.
Improve the thesis, put your version on your webpage, and inform the advisor that an error was present, which is corrected a posteriori. That's it; you've made your best to improve, and now it's the advisor's task to grade you.