If you enter World 7-5 while wearing a Frog Suit and don't lose it early in the stage, you'll eventually reach an area from which you cannot go any farther. You'll fall through a hole in the floor and be trapped on the left by a wall of bricks you can't slide under while wearing a Frog Suit, and a wall of ice blocks on the right you can't pick up while wearing a Frog Suit, either. You can't jump back out the hole you fell through because invisible blocks appear to bar the passage. The only way out of this mess is to let the timer run out.
Taken with permission from Flying Omelette.