As many have already correctly pointed out, in the case of open-source software, any skilled programmer can present their version of a fix to a newly-discovered bug or error by making a pull request. But you don't have to be a skilled programmer or a person able to read and understand code to find a bug in the software. If, upon interacting with bitcoin, you encounter a problem never before seen, you can open an issue on GitHub or a similar platform, where you will describe an error and all the steps that lead to that error. This way, you can help developers to find a solution to the problem. It is also important to note that some errors in bitcoin cannot be fixed except through making a hardfork, that is, making bitcoin incompatible with current nodes' software. This kind of bug remains unfixed because their fix may be more harmful than the errors themselves.