There are currently also a few more not-obviously-localized forms of undefined behavior, even in the C standard. In N3096, I found

restrict seems tricky, buit can probably be handled like data races?

"The execution of a non-directive preprocessing directive results in undefined behavior."

Referring to signal(): "Use of this function in a multi-threaded program results in undefined behavior." Actually, I don't understand why that's there at all. I could see "implementation defined". As it stands, if I call signal(), and then create threads, it encounters undefined behavior at the first thread creation? Do we guarantee that the standard library doesn't create threads behind the scenes?

H.13: "Use of the macro with an argument of a decimal floating type results in undefined behavior." This seems likely to be diagnosed way ahead of executing it.

A bunch of the J.2 undefined behaviors, like "A nonempty source file does not end in a new-line character which is not immediately preceded by a backslash character or ends in a partial preprocessing token or comment (5.1.1.2)." are purely static code properties.

Hans