Just found an amusing quirk whilst trying to add a button link to a drop down menu item (creating a basket drop down so I wanted a nice big button to direct users to go to the checkout page rather than a standard link).
If you add a button inside a drop down's 'Item' it's possible to set the button to a link type which is illegal in html. Admittedly pretty much every browser will consider the outer tag to be unclosed and so add the closing tag. Meaning this isn't much of an issue for rendering but it just threw me as to what was happening.
(Subsequently removed the button and just added the button classes to the menu link instead but thought it might be a quirk worth being aware of).