After installing "plain_site", the frontpage does not validate. Tested in 3.10 and 4.0, but is probably true for earlier versions as well. I did not test with other install options (Website Interface, eZ Flow), they might also be affected.

The problem was originally reported here:
http://ez.no/doc/ez_publish/technical_manual/4_0/templates/the_pagelayout#comment19853

Steps to reproduce

Install "plain_site" in 3.10 or 4.0, open the publicly available frontpage in Opera and press CTRL+ALT+V.