User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.3a4pre) Gecko/20100403 Ubuntu/10.04 (lucid) Minefield/3.7a4pre Build Identifier: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.3a4pre) Gecko/20100403 Ubuntu/10.04 (lucid) Minefield/3.7a4pre When creating and appending a text node to the body of a page within a repeated setInterval, using an un-passed argument (which should evaluate to undefined), the text node contains random numbers, mostly zero, instead of undefined. Opera and Chrome do not show this behaviour. Reproducible: Always Steps to Reproduce: 1. Create a setInterval call 2. Make the function have an expected argument defined 3. Add a createTextNode/appendChild call including that argument Actual Results: Random integers are written over the screen, mostly zero. Expected Results: The word undefined should be written repeatedly over the screen. This is the offending code: setInterval(function(t) { document.body.appendChild(document.createTextNode(t + ' ')); }, 0); More details at my blog post: http://azabani.com/45

It's not exactly random, see https://developer.mozilla.org/en/DOM/window.setInterval#Callback_arguments This is tracked in bug 394769.