In all seriousness, sonofjones, I'd personally react with "argh" for both pain and shock (which is what these two instances appear to be combinations of, though it's possible there's no pain involved in either). Or alternatively I don't react at all because i'm just that badass.

Everyone is different.

Some people might go "Shiiiiiiiiiiit" as they fall and "owwww" as they're hit in the face with steam.

I still don't see it's as obvious as you apparently think it is which noise should accompany which action. Both seem appropriate and as far as i'm concerned could be interchangeable.