I'm having the same problem. Clicking "[mark as not studied]" in http://popupchinese.com/lessons?show=studied and "mark as unstudied (grey text)" in individual lesson pages don't seem to work.

I have even cleared my cache, nothing seems to work.

I'm running Windows 7 64-bit.

Tried it on Chrome 20.0.1132.57 and Firefox 11.0.