2 401 errors then image loads!

[Anonymous Poster]

We have noticed a weird phenomenon with images rendered on an asp.net page
(in this case from a MS TabStrip control). On each postback (and on the
initial page load) it tries to load the image twice and fails with a 401
error before succeeding (with a 200 or 304 if its cached). Using Fiddler
(http monitor) produces the following output -

session # Result Host Url
5 401 localhost
/SDA/WebApplication1/UserControls/images/tabspc.gif
6 401 localhost
/SDA/WebApplication1/UserControls/images/tabspc.gif
7 200 localhost
/SDA/WebApplication1/UserControls/images/tabspc.gif

It is always 2 failures then success. The 401 page is 4k so its wasting 8k
each time (and thats just for one image!). The entire site is anonymous
access. What is causing this and how can we prevent it happening?

thanks,

Simon