You can do a variety of things to remedy this:
- If the OP is relatively active, then comment on the post, and let him/her know the link is dead
- If you do not receive a reply, you can try to find a link that is similar to it, and if you are sure, change the link in the post
- If you are unable to find a similar link, and a link is to a webpage whose server is no longer serving it or if the server is down for extended periods of time, try finding cached versions of the webpage using the WayBack Machine or Google's cache.
Here are some common errors that you can come across and what to do about them
500-(Internal Server Error): This is probably a temporary error. Comments will only be required if it's been days and the same error recurs
404-(Page Not Found Error): One of the most common errors you will find, it's probably indicative that the page is gone forever/moved. Try finding the page on the parent website. If you can't just use the WayBack machine