Just stumbled on the following hack in an nginx error page:

<!-- a padding to disable MSIE and Chrome friendly error page -->
<!-- a padding to disable MSIE and Chrome friendly error page -->
<!-- a padding to disable MSIE and Chrome friendly error page -->
<!-- a padding to disable MSIE and Chrome friendly error page -->
<!-- a padding to disable MSIE and Chrome friendly error page -->
<!-- a padding to disable MSIE and Chrome friendly error page -->

These comments are hard-coded in nginx and just expand the response length. Apparently, some browsers (IE and Chrome, obviously) would replace the error page with their own if the response is below a certain threshold (512 bytes, probably?). And the minimalistic error pages generated by nginx can easily fall below that.