The error is

"a[3] is undefined"

. As you pointed out,

   $('.box(:not:has(:radio:checked))')

is not valid syntax. jQuery doesn't try to give a nice error message for arbitrary selector syntax errors. I'll recategorize this as a enhancement request for better error handling.

I had a lot of problems in IE upgrading from 1.2.6 to 1.3.0 as the following statement did not result in "Syntax error, unrecognized expression: @href^=http" being shown when the following statement executed.

$("a[@href^=http]").not("a[@href^=http://"+location.hostname+"]").each(

// blah

)

Changing

throw "Syntax error, unrecognized expression: " + expr;

to

alert("Syntax error, unrecognized expression: " + expr);

allowed me to see the error and easily fix it by removing the @'s.

But why did I get an uncaught throw error in the first place?

@WanderingZombie, something was probably catching and discarding the error via try/catch, but not having the code it's impossible to tell.

Current versions of Sizzle don’t throw errors and just return an object with zero elements. Not sure if that is intentional or not.

As we move towards using the browser's native querySelectorAll implementations, there won't be a lot we can do to improve error messages in Sizzle. It might be possible to do better error checking via a plugin but I don't think we'd put it in the production code.