I had to look this up. It seems he has a periodic table with lots of little cubbyholes with pictures and items representing the element, but not every element is actually there.
It's hard to find a good source for details about this display, but he has cubbyholes for all of the lanthanides and actinides, so there must be some that don't have the actual element being represented.
Francium is the one I always think about. My understanding is the only source is as a decay product (probably from uranium) and that there's somewhere on the order of one ounce of it in the entire planet at any given time.
Not only that, but it's only true francium if it comes from France.
It's hard to find a good source for details about this display, but he has cubbyholes for all of the lanthanides and actinides, so there must be some that don't have the actual element being represented.