Surprise attacks

Surprise Attack Zombies are Zombies that suddenly appear in all stages except for Day. The Surprise Attack Zombies can be called as Ambush Zombies and usually comes in sets of 3, with the exception of Night levels, as there are always more than 3 graves. The Ambush Zombies can be normal Zombies, Conehead Zombies, Buckethead Zombies and their Ducky Tube Zombie form.

Strategy

 * For Night Ambush Zombies, just use Grave Busters to destroy the Graves before the Final Wave.
 * For Pool and Fog Ambush Zombies, when playing to the final wave, wake up/use an Ice-shroom right after the wave comes but before the Ambush Zombies pop up from the pool. Another way is to plant a Cherry Bomb/Doom-shroom in the middle of the Ambush Zombies.
 * For Roof Ambush Zombies, they are dropped by Bungee Zombies and can be stopped by planting Umbrella Leaves.

Trivia

 * Ambush Zombies in Pool and Fog levels have seaweeds on them.
 * The seaweed patterns are always random.
 * It is exactly unknown how the ambush zombies from Pool of Fog levels have seaweed on them. It is possible the pool could have algae in it, but that still won't explain the green clumps.
 * Maybe the seaweed could have came from the Tangle Kelp when they were pulled down, but the seaweed appeared before Tangle Kelp was unlocked.
 * Ambush Zombies have three Mini-Games featuring them: Grave Danger (which is a hidden Mini-game), Whack a Zombie and Bungee Blitz (which is also a hidden Mini-game).
 * It is also unknown how the zombies got there in the first place.
 * Maybe they got stuck in there while you fought them in Day and Night.
 * The Day levels are the only levels that don't have any surprise attacks.
 * In 3-1, 3-2, 4-1, 4-2, 5-1, 5-2, there are only two Ambush Zombies.