seventh dwarf phenomenon
When you can't remember the last item in a group or list because you can only think of the ones you have already named, just as it's really tricky to remember the 7th dwarf's name, no matter which 6 you name first.
I know there are 7 -stan countries, Pakistan, Afghanistan, Tajikistan, Uzbekistan, Kyrgistan, Tajikistan, and I can't think of the last one. It's the fucking seventh dwarf phenomenon.