At the start of Chapter 2-2, you will come across a character called Mimi. She will tell you to go to the farthest room on the first floor to meet a character called Merlee. Enter that room and press the green switch. A trap will be triggered, and a spiked ceiling will descend upon Mario. Before the ceiling reaches him, flip into 3D and jump on it to find a key.

If you pick up the key, you won't run into any problems. Use it to open the locked door to Mimi's right. However, if you talk to Mimi without picking up the key, the game will freeze. You will then have to turn off your Wii console and start again from your last save point.

Via Nintendo Europe