Suppose you're using the W3C validator to validate HTML5. Here's a sample document:
<html lang="en" xml:lang="en">
<meta charset="utf-8" />
var foo = 1;
Everything looks right, but you get this mysterious error:
Line 6, Column 48: Element script is missing required attribute src.
(Note: a bug has been filed and this will be fixed in a future release.)