I added a new custom user field in the "Preferences" location. I added a regular expression match. Now, when someone edits their preferences, and they fail to match the regex they are taken to an error page (Please enter a value that matches the required format.). That's fine.
Now, when you open up preferences again, to try to fix the problem, if you enter a regex that matches correctly this time the value is changed as expected, but instead of going to a valid page you're given another error page. This time it says "This action is available via POST only. Please press the back button and try again.".
I guess it has something to do with being on an error page when you edit your preferences and when it tries to reload that page after editing your preferences that's where the error comes from. A solution could be to redirect the user to the home page if they edit their preferences when they're currently viewing an error page.