The Home Office recently introduced a set of new validity requirements for visa applications under the Points Based Immigration System, such as the Skilled Worker route. This is significant because an invalid application doesn’t extend your permission to be in the UK while it is being considered (what’s called “section 3C leave”). So if you dutifully apply to extend your visa just before it expires, and the application is later rejected as invalid because you failed to meet a validity requirement, it will be like you never made an application. You’re now an overstayer. Validity requirements existed before but pretty much all the old ones (in paragraph 34 of the Immigration Rules, which still applies to non-Points Based Immigration System applications) were things that could be rectified. If you paid the wrong application fee, for example, the Home Office would send you a letter giving you two weeks to pay the correct fee. If you did that, your application was validated and considered valid from the date you lodged the application, so you had section 3C leave all along.