When using Builder to construct table elements (including TD and TR), IE6 throws an error and stops script execution.
The problem: IE6 considers element.innerHTML to be a read-only property for the elements COL, COLGROUP, FRAMESET, HTML, STYLE, TABLE, TBODY, TFOOT, THEAD, TITLE, and TR (see this discussion).
Builder.node() tries to create its elements by first creating a parent element, then inserting the desired element into the parent via assignment to innerHTML, then retrieving it via parentElt.firstChild(). I don't know why; legacy support, maybe?
Thus, when using Builder.node() to construct, say, a TR, Builder creates a TABLE then tries to assign to its innerHTML. Kaboom!
The fix: wrap both instances of "parentElement.innerHTML = something" in a try block. These are on lines 30 and 51 of builder.js in 1.5rc4. The catch block can be empty; if the innerHTML thing fails, Builder goes on to try a simple document.createElement().
I've tested this on Safari 2.0.1, Firefox 1.5b2 on OSX, and IE6 on XP sp2. Sorry for the lack of a patch; I don't have SVN.