Lightbox JS and HTTPS
I've always loved Lightbox - it just looks so cool. I've used it to "popup" larger versions of images on a few sites, but only today did I notice that it causes a security warning in IE7.
Unfortunately, this isn't highlighted as an issue on the lightbox page, so I didn't know about it - and this can cause cart abandonment if it happens during the checkout process!
The fix was easy to find on their forums, but I would have preferred to have known about it in advance 
So, here's your warning - if you use lightbox or are about to, follow this advice.