How to reproduce:

Edit a host, get a conflict: this displays an error info in a modal dialog.
Ack then overwrite: it should have shown another error info in a modal dialog too,
but instead of an HTML fragment, foreman gave a full HTML page (with html, head and body) as an answer to some AJAX query,
this has the nasty effect of clearing the page, only showing "2012".

Clue:

$.ajax({
  [...],
  success: function(result) {
    # If result is an HTML fragment, everything is fine
    # if it is a full HTML page, this will somehow clear the page and show "2012"...
    $('#modal-frame-content').html(result);
  }
});

This is a general problem that can arise with code like the above.