The worst is when you discover this after you already posted! If it's not too many things to change, I go back and make the corrections. If it's huge (like changing tense midway) I make a note to fix it at a later point. Several of my WIPs are being reedited as I post them a chapter at a time at new archives, and this gives me the chance to repair mistakes of this kind. To prevent them, I self-edit continuously as I write!