Site crashes when I try to modify functions.php – help!

When i edit the parent theme (twentytwelve) and hit “update” my site will crash. It won’t allow me to update it even when i DONT add/change any text in function.php. What the heck?

In fact, this happens with multiple sites of mine, all with different wordpress and theme versions. What gives?

I have to manually edit the functions.php file in Dreamweaver then re-upload.

All files are writable via FTP.

PS. my goal is to understand why I can’t do this, not to get a lecture on using child themes.

Thanks!