Why they decided to make Joseph a red ball with legs and Kars a hat-wearing pink thing, we will probably never know.
A completionist's worst nightmare: It's rather frustrating to play through a whole game, only to learn about an item or achievement that was only accessible at a certain section and gets locked out afterwards. Meaning if a player wants to re-access it, they have to start from the beginning on a new save - or even delete the current one that they got attached to.
Example #1: Pikuniku: Certain collectables and achievements could only be collected through one-time events, such as the "Dance Machine" achievement in the dance battle segment. Special mention goes to the Mr. Sunshine trophy, which >!can only be found in an easily-missed corner at the volcano, which cannot be reentered afterwards (the x-ray goggles can make it easier to find)!<.
Example #2: Kingdom Hearts I: In the game's original release, one of the collectable Trinity Marks can only be accessed in a room at Oogie's Manor. Once the manor is destroyed, the Trinity will also disappear if uncollected, making 100% Completion impossible. Fortunately, rereleases of the game have moved the mark to somewhere outside of the manor, making it accessible even after Oogie's defeat.
Example #3: Metroid Prime: Several scannable items and enemies, such as the maps and Parasites, can no longer be scanned after progressing past their areas or defeating them. Though the Hard level does allow another chance to scan certain missed ones.