Some of my old srcipts I used dollar sign ($) as a function name.  This is normally not a problem until you start using prototype or jquery.  If you are using either one of these 2, you will need to check that you don't use or have any functions with a dollar sign ($) as the function or value name. It took me a little while to find what the problem was as firebug didn't display any problems, just that jquery wouldn't work.

I also found that using a dollar sign as a js function name also causes another problem within IE6. If you have a function ie. function $(sender) and you are running IE6, when you close IE6 it will come up with a scripting error. This is the dollar sign JS function causing this. Rename the $() function and this will stop.