Sitting upside down will break the avatar, world, and social menus. Any scrolling lists will break entirely and allow you to either scroll forever or not display anything at all.
Steps to reproduce:
Sit in an upside down chair and open the menu. Depending on how far the chair is rotated the menu will break a little less.
I used Room of the Rain and placed a movable seat upside down.
(Not the greatest of quality)
I am very confuse...