I continue to get this error as well. And it appears to have no relationship to file size. The only way I can consistently reproduce the error is when the page the upload control is located on is accessed from a Response.Redirect("pagewithcontrol.aspx").

 

If I ask the page through a traditional HTML link, the error is not thrown.

 

It also doesn't appear to be a caching issue of the page as I append a unique querystring to the url. Any ideas around this?