PHP script is returning "Save file as..." dialog (delete confirmation form)

One of our users is reporting that sometimes when she tries to delete a comment (path comment/delete/%cid) it would not work but she would get an error page. After providing a screenshot, it turned out that the "error page" actually is the PHP script not executing but instead making the browser to open a "Save file as..." dialog:

Screenshot here...

Can anyone help with this?