See the URL: I've intentionally added }} to one of inputs, which resulted in template being prematurely closed and missing several parameters. While users certainly certainly should be able to insert all kinds of wikitext (including templates and HTML tags), the wizard could at least check for mismatching elements.

Browser: Chrome 6.0.466.0 dev

Version: unspecified
Severity: enhancement
URL: http://commons.prototype.wikimedia.org/w/index.php?title=File:Testuserrrr_1280415964617_A_JPEG_2000_file.jpg&oldid=367718004