On this page: http://www.w3.org/TR/WCAG20-TECHS/ARIA11.html#ARIA11-ex4

Example 4 shows a role="search" on a form:
[form role="search"]

Doing this will cause the validator to throw a warning. I also understand (thanks to chats on the Gitter channel) that it is not good to apply this role directly to the form as it will override native semantics. It should be applied to a container element instead.