Everyone who's nailed McCain for confusing Sunni and Shiite is absolutely right.  But have a look at this article from The Guardian.  Worryingly, religious distinctions seemingly closer to home give him equal trouble. 

Durie also found other inaccuracies in extracts from the McCain memoir. For example, the suggestion that McCain's grandparents on his mother's side were descendants of Scots Presbyterians who "suffered the privations that were the fate of those who had remained loyal to the Scottish crown" in the aftermath of the death of Queen Mary.

"Mary Queen of Scots was Catholic. She was executed in 1587 by which time the Presbyterians were well in charge, so why would there be 'privations'? This is a mangling of history, or at best a misapprehension of it," Durie said.

Sunni/Shiite.  Catholic/Protestant.  Whatever.