I'm not sure this is worthy of a post, but it's an issue that I couldn't find a solution to on the web, so may as well fill the gap.
If you're using the Thickbox JavaScript library and run into this error: "imageGroup is undefined", the solution is simple: Add a rel attribute to your <a> tags that wrap the images.
<a href="..." rel="group1">
If you're interested, the error is sourced from this line:
var imageGroup = jQuery("a[rel='"+rel+"']").get();
Essentially, it's expecting a rel attribute, but doesn't properly handle the case where it might not exist.