I disagree with the above responses: I have had very bad times hunting through apparent javascript bugs before I remembered to validate the page I was working on. As soon as I corrected the glaring errors, the scripts worked as intended.
The most obvious problem (not one I have experienced, because I already look for that): have two elements with the same id attribute, and try to make your script that depends on that to work as intended :)
While it may be true that the level of validation needed is somewhat low, when you are banging your head against the table because your website doesn't work and you don't know why, you'll remember this fondly.