I’ll kick things off with a quickie:

PROBLEM:
You’re trying to use jQuery in your WordPress theme, widget or plugin but you keep getting an error that’s something along the lines of “Result of expression ‘$’ [undefined].”

DIAGNOSIS:
Chances are, there are other JavaScript libraries being loaded that also uses “$” and it’s fucking up your juice.

SOLUTION:
Use “.noConflict()” to get your party started.

$j=jQuery.noConflict();
$j(document).ready(function($){
// Code that uses jQuery's regular "$" can hang out in here.
}