Sweetie Belle's 5th birthday

On Sweetie Belle's fifth birthday, she was upset to see what appears to be Rarity stealing the attention from everypony, which left her upset. Luna then shows Sweetie what really happened during the time she was getting ready: the guests were getting bored and Rarity tried to stop them from leaving by letting them play with the favors early. In other words, Rarity wasn't trying to steal the spotlight, she was trying to save Sweetie's party, and she didn't even know it.

Example of:
Once More, with Clarity