When you are required to have a certain item or power-up to reach a collectible, especially when said item cannot be found in the level you’re playing, so you have to return to an earlier level just to ‘’harvest’’ items and then return to the level the collectible is located in, or when you need to carry an item through much of the level to use it where required.
This is why it took me so damn long to collect every Star Coin in New Super Mario Bros. (Returning to 1-4 over and over again to get Mini Mushrooms was annoying as heck.)
This is why it took me so damn long to collect every Star Coin in New Super Mario Bros. (Returning to 1-4 over and over again to get Mini Mushrooms was annoying as heck.)