Open the new_issue page and go do something else for... a while. I don't know what the limit is, but after a certain period of time clicking the Create button simply erases the text you entered in the Description field. The Title field is preserved.

Fortunately, for me at least the text comes back when I use the back button. But that may be some firefox extension I have installed. In order to actually create the issue, you have to save off the text and go back to the point where you can click the "New Issue" button.