In some games, when you miss a pickup, you can never get it again unless you restart the game, and those pickups count in your collection. (therefore, you must restart the game to finish the collection, if you made any mistakes).
In Pokémon, you have only one Master Ball, limited supply of Ethers and legendary Pokémon appear once per game. Trading fixes all of those problems.
For a more radical perfectionist, they demanded all dialogs, background music, and even items to be revisitable. Some games, like Naruto Ninja Council, satisfied all of those needs.
Is it always a good practice to make things revisitable? Even cutscenes and music? When is it not a good practice to do so?