It is kind of annoying when it happen
used to copy the text before sending it or in my phone i used to connect with uc browser which you can recover the text when you press the back button but it is still a little buggy
That or you can just hit preview and it's saved in 'draft' for 7 days (draft is right under preview. Go ahead, give it a try!) if you're writing up a good in-depth post just make sure you hit preview and you're good to go.
I get these 502 errors at least once a week and I find that after a few refreshes it's usually resolved. Once in a blue moon it really affects me and I can't do anything at all on bitcointalk for a few hours. That hasn't happened in months and months though.