Just stumbled across wtfjs.com, and would like to submit this classic, specific to IE.
This must be an oldie but always astonishes me when I trip over it. In your browser, try:
var ar = [ "cat", "dog", "fish", ]; alert(ar.length);
In most browsers it displays 3. In IE8 it displays 4. It’s the trailing comma in the array literal. I think ECMAScript says this is a syntax error, but IE’s behaviour here is extraordinary nonetheless.