Here are a few things I learned from reading the jQuery source code. I’m documenting them here because they’re edge cases that are, in some cases, difficult to reproduce and debug. The jQuery source code is very clean and well commented and has many more gems than what is listed here.

getElementById sometimes returns elements by name in some versions of IE and Opera.

                                // HANDLE: $("#id")
                                } else {
                                        elem = document.getElementById( match[2] );

                                        // Check parentNode to catch when Blackberry 4.6 returns
                                        // nodes that are no longer in the document #6963
                                        if ( elem && elem.parentNode ) {
                                                // Handle the case where IE and Opera return items
                                                // by name instead of ID
                                                if ( elem.id !== match[2] ) {
                                                        return rootjQuery.find( selector );
                                                }

                                                // Otherwise, we inject the element directly into the jQuery object
                                                this.length = 1;
                                                this[0] = elem;
                                        }

                                        this.context = document;
                                        this.selector = selector;
                                        return this;
                                }

Even something as simple as getElementById can have edge cases you have to take into account. In IE 6 and 7 for Windows and some versions of Opera, dom elements can be returned by name instead of id. In addition, as seen in the comments of the code above, Blackberry 4.6 can return nodes that are not even in the document.

See references here and here.

A false positive on the domready event can cause rapid page refreshing.

        // Handle when the DOM is ready
        ready: function( wait ) {
                // Either a released hold or an DOMready/load event and not yet ready
                if ( (wait === true && !--jQuery.readyWait) || (wait !== true && !jQuery.isReady) ) {
                        // Make sure body exists, at least, in case IE gets a little overzealous (ticket #5443).
                        if ( !document.body ) {
                                return setTimeout( jQuery.ready, 1 );
                        }

The domready event, which is present in some form or another in many browsers can fire before the dom is actually loaded. This can cause continuous page reloads and other errors which are hard to reproduce and debug. The solution to this is to check that the document.body is not null before accepting the domready event.

Of course you can always fall back to the window.onload event, however this will force you to wait until the entire content of the page (including possibly large images) has loaded.

See references here and here.