I don't have those boxes normally unchecked. And I don't remember unchecking them.

I've seen a bug like that on a Perl website which was using the POST method. It can happen sporadically when the software does not check whether the amount of data received matches what CONTENT_LENGTH said should be sent. (Perl's CGI.pm only remembered to do that on multi-part posts. I sent in a bug report. I'm not sure that it was acted on though.)

Cheers,
Ben