Take a simple page
Textbox + requiredfield validator
InputFile
Progressbar
UploadButton
CancelButton - causesValidation is set to false
with no text in the textbox, click upload, validation fires and prevents postback
still with no text, click cancel, nothing happens
add some text, click cancel, page posts
Having a file selected in the InputFile makes no difference to the behavoir
if useHttpModule=false is used, cancel button works
One can only assume that there is some sort of interference with the asp javascript.