You can repeatedly clone three Mushroom Blocks in World 6-1's vase room with this neat glitch. The room starts out with Mushroom Blocks covering the 3rd, 13th, and 21st (last) vases. If you move one or more of these three Mushroom Blocks and then go down certain vases, when you come back up new blocks will have replaced any of the three that were moved. You can repeat this over and over to fill up the room with Mushroom Blocks! (Note: Don't go down the the 3rd, 7th, 10th, or 17th vases—these lead to digging areas and will reset the room when you come back out, removing any cloned blocks you made.) Tip: Since Cobrats lie in wait under the first and last Mushroom Blocks, the quickest (and safest) method is to move the middle Mushroom Block (13th vase) and then go down one of the vases that was next to it.

This bug is not present in the NES or GBA versions of Super Mario Bros. 2.