Document.write in a block breaks stuff when IPE selected as renderer

To reproduce:
- Add content to a region. The content should contain something like <script>document.write("trololo");</script> and the page's renderer should be set to Standard.
- View page, see "trololo" printed in the region where the content was placed.
- Change renderer to IPE
- View page, see white page with the whole DOM replaced with just the "trololo"

I think I saw a similar issue in ctools's queue but can't find it now. Also I think it wasn't IPE specific.