You can trivially farm data that people are sending to their local apps by having DNS occasionally change to an IP you own (and then immediately back, to avoid detection). On that IP, you would have something that remembers the parts of the request that might be interesting (query parameters, body) and return a benign-looking error (just severing the connection without a response would probably do it). At the very least, you'd probably get some email/pass pairs that work on a few major sites.

From your browser's perspective, the POST would fail, if it expected a certain response. That would happen after the site already recorded the contents of your POST.