We have found that the first file upload after a Javascript call to Page_ClientValidate which returns false will not fire the OnUploadedComplete event. 

 

Scenarios extensions:

1) A second upload after the Page_ClientValidate function will invoke the OnUploadedComplete event.  It appears to be only the first upload after a Page_ClientValidate call that returns false to be the issue. 

2) After the OnUploadComplete fails to fire (after a page validation issue), if the validation issue is fixed and the Page_ClientValidate function is invoked again, the OnUploadComplete event then fires without uploading another file.

3) If the page validates fine, the OnUploadedComplete event fires normally...

 

This has been reproduced in FF 3.5 and IE 7 on Vista (currently under the Visual Studio 2008 web server).  Also reproduces under IIS on Windows 2003.