Saki's Cosmosphere, Level 8
Level 8 Cosmospheres are the no-no lands for a 2:1 path, especially for Tyria and Cocona's endings (Cocona's especially). As such, this and the following can be missed for good on one playthrough:
Despite saying that, this technically can't be missed if you are going for a complete Saki CS run. You must either accept Saki in a 2:1 playthrough, or have given her all three heart points (and, of course, have the necessary talk topics to even access the 8th CS level). Through regaining Saki's physical form, you will end up birthing a new Will of the Planet to take Saki's place by the name of Yea.