Basically what is happening is that (in Chrome) clicking on an image thumbnail opens the lightbox as expected, but it also opens a larger image in a lightbox over it. This means that a user has to click once to close the first lightbox before he/she can use teh lightbox as intended.

An example sample can be seen here: http://www.ladidaladida.com/inside-the-store

Here is a 10 second screencast to illustrate: http://screencast.com/t/OCKUySGelk

Note: I've only tested on Chrome on OSX Lion