I feel pretty confident in saying that at least some poorly-written web apps will end up sending new status codes to user agents that are not expecting them. I

I feel pretty confident in saying that at least some poorly-written web apps will end up sending new status codes to user agents that are not expecting them. I also feel pretty confident in saying that it will be the browsers that will be perceived to be "in the wrong" when they reject or ignore such responses, even though it's solely the fault of bad web apps. (See how browsers today need to accept and try to correctly handle totally broken and incorrect markup, for instance.)