odd 404 error page behaviour in IE9

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Hi,

running IE9 on win7 64 bit, if i go to :
http://www.concrete5.org/
and mistype a page eg:
http://www.concrete5.org/wibble...
i DONT get a 404 error....
i get a page fragment as shown in the image:
this only happens with IE9 not chrome or safari.
it happens on ANY concrete site ive tried....
anyone else replicate this issue???

is it IE9 or concrete issue??

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