When does Internet Explorer forget it has cached an image?

Go here again (don't refresh)

First row (: url(image1.png))

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Second row (: url(/stuff/css/ie-caching-images/image2.png))

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Third row (: url(image3))

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Fourth row (: url(./image4))

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Some access.log information

After visiting this page and clicking the 'Go here again' link this is what I got.

Opera 8:

"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image1.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image2.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image3.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image4.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Opera/8.01 (Windows NT 5.1; U; en)"

Firefox 1:

"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image1.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image2.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image3.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image4.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.10) Gecko/20050716 Firefox/1.0.6"

Internet Explorer 6:

"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image1.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image2.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4.png HTTP/1.1" 304 - "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 2319 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"

Hmmm, a missing extension causes this?

"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 7778 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 7778 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 7778 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 7778 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/ HTTP/1.1" 200 7778 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image3 HTTP/1.1" 200 268 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"
"GET /stuff/css/ie-caching-images/image4 HTTP/1.1" 200 259 "http://ktk.xs4all.nl/stuff/css/ie-caching-images/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)"

Gotta look more into this.

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