The mushroom houses at the start of each world (the point with an arrow at the start) differ determining on the time you get on a stage. If you get x44, x55, x66 on the timer, a Green Mushroom house appears, which is a 1up minigame.
BONUS TIP: In World 7-5, throw a Koopa shell in-between the two Bullet Bill cannons to get an extra life every four seconds

:)