This is the offending line in the built jquery.js:

1. // Override the abort handler, if we can (IE doesn't allow it, but that's OK)
2. // Opera doesn't fire onreadystatechange at all on abort
3. try {
4.     var oldAbort = xhr.abort;
5.     xhr.abort = function() {
6.         if ( xhr ) {
7.             oldAbort.call( xhr ); // <-- * DIES HERE *
8.         }
9.          onreadystatechange( "abort" );
10.     };
11. } catch(e) { }

The error only happens in IE7. IE8 and FF 3/3.5/3.6 are all unaffected. If I comment out just this single line, IE7 behaves correctly.

The pile of code this call could be coming from is quite large so I'm still trying to track down exactly what request is triggering the error, but might anyone have any ideas off the top of their head?

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$("#BenjaminPottle")