TODO:
You have an Application Page, or regular ASPX page, .Net 3.5 and you notice that each Postback is happening twice, for no apparent reason.
SOLUTION:
If you have an IMG SRC="" tag, or a placeholder like IMG SRC="#" this will cause the issue. If dynamically generating SRC values, be sure that it is actually being populated with a valid image path.
NOTES:
This bug has been fixed in .Net 4.0+