hi,
follow the steps below,i'm sure u can reproduce the issue:
1,open the url
http://jsbin.com/ajuji4 in firefox
2,open the firebug console window
3,reload the page
4,mouse over and out for several times on the dark-gray div with text 'div#testDiv for test'
or 5, just open the link in ie browser with the option 'disable script debugging' off.
btw, i attached a print screen image.
IMHO,prototype.js extends the 'each' method on the raw javascript Array object,which maybe lead to this issue.
thanks again.