Typing the following in a richtext fieldtype:

• A
  • B <enter here a line break (SHIFT-ENTER)>
    C

Causes the following message on "publish" (not save):

Validation of XML content failed: Error in 344:0: Element section has extra content: itemizedlist

To reproduce you need to have more than one level of bullet (or ordered) list.

The following works:

• A <enter here a line break (SHIFT-ENTER)>
  B