i think you are not reproducing properly as i could reproduce in your example as you need to just keep refreshing and submitting the page it will popup  i have taken the screen shot  of that error using your example. mine browser is  same as yours.

Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1.

 

looking for positive response.