In Worlds 1, 3, 5, and 6, you can turn the wandering Hammer Bros. enemy on the map into a treasure ship loaded with coins. To do so, you must complete a stage while the following is true:

The stage timer has an even number of seconds remaining
Coins collected are a multiple of 11 (i.e. 33, 44, or 55)

The 10s digit of your score matches the digit present in your coin total (for example, you have 11 coins and the 10s digit of your total score is a 1)
If all of the above is true as you collect the card and complete a stage, a Hammer Bros. enemy will turn into the treasure ship, and you have one attempt at the stage. You can collect a bunch of coins, and will face an enemy at the end.