Links to works that don't yet have pages will thus show up as red. However, they'll automatically shift to blue when the page is created, so always link to works even if they don't have a page yet, unless the work would violate The Content Policy—it makes it easier for when someone eventually writes it up. We do make an exception for long titles over the 64-character limit mentioned on How to Create a Work Page — red links that are too long come with some technical problems, so they should either be linked with an official shorter title or not linked at all.
A misspelled or otherwise botched link will be red too, even if the page it was intended to go to does exist. These might be Frequent Redlinks, and should be fixed on sight.
Pages destined to forever be redlinks are listed on Permanent Red Link Club.
Sometimes, Tropers will make redlinks without even intending to link! This is because the markup assumes every use of CamelCase is supposed to be a link. This can be a pain when trying to talk about, say, the Speed Chat function in an online game. In order to fix links like that, put [=this=] markup around the word, so Speed Chat becomes SpeedChat.