Or the instance you're on goes down for a minute (maintenance or the server gets overloaded).
Do what we used to do back in the day, type your long posts in notepad/textedit/whatever, then copy them over. The Lemmy post editor is pretty crappy in fact, with the URL constantly updating as you type, not sure what that's about but the flickering of the URL bar annoys me, so if it's more than a couple lines, I'll just open textedit, type there, then copy/paste over. If the site is acting up, well, I still have my text.