VIA Pinhacks:

During attract mode, hold both flippers down for over 30 seconds. If you don't hold them down long enough then you can't try this again right away. I'm not sure if a timer has to reset, or if a game as to play and end (or test mode enter/exit).

After you let go of both flipper buttons, at a relatively quick and constant clip enter the following flipper button presses:
10 Lefts
10 Rights
2 Lefts
2 Rights

After the above sequence, code at $6948,37 (ROM Addr 0x5e948) jumps to code at $8D8C (ROM Addr 0x78d8c) which is the power-up vector! This is the address that CPU normally automatically jumps to at power-up (same as the last 2 bytes in the ROM 0x8D8C).

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Fun, but not related to the bad Easter Egg:

In the Secret Ball Lock animation, you can spot DOHO in the lower left corner.

When you start a game, hold the left flipper button. The first Basement Award will now be Digital Pinball where player 1 plays for 40M, all other players play for Extra Ball.

Holding the right flipper button at the start of a game will activate MK3 Symbols. Spell THEATRE to reveal the symbols.

During the Metamorphosis mode, the woman sometimes transforms into a cow. In addition to this, you can also see the woman transform into the snack-bar guy from Creature From the Black Lagoon.
In this case, he says "Here's your hot dog!"

Shoot the Trunk three times in Hat Magic mode to make a telephone appear on the screen. "Move your car" is heard from it.