See also: commons:com:Forum#Upload wizzard

I and a number of regular uploaders experience failed uploads with typically the upload time increasing instead of decreasing during the upload and often terminating with the error message unkown error $1

It would absolutly be helpful, if there was a more specific error message than unknown error and if the variable $1 would always be instantiated with at least something.

This is a massive time killer and leaves so many broken uploads in the upload stash, that the special:uploadstash page will run into the time limit, only showing wikipedia is expiriencing problems, come back in 22nd century.