<p>I actually had that problem last night even, and all I did was log out of Common App, exit out of my browser, wait a while, and try again. Seemed to work alright, but today could be different I suppose.</p>
<p>I've worked as a professional web application developer for several years.</p>
<p>Even if you missed a deadline because of an issue like this, there is probably evidence in log files kept by the web server that you attempted to submit the supplement and it failed. In a worst-case scenario, this would prove that you missed the deadline through no fault of your own.</p>
<p>It's still probably better to try again every couple of hours until it works. These problems are often sporadic. Also, they know people have December 31 deadlines and will call someone in to fix it, even if it's a holiday.</p>