Port Overdosed: Both the original Bubble Bobble and some games in the Puzzle Bobble/Bust-a-Move series are exceptionally widespread games, having appeared on virtually everything able to play games since their initial releases—which includes, in Bubble Bobble's case, every home computer of the 1980s.
Prop Recycling (sort of): Curiously, the bubbles around the player characters (used for level transitions) in the two games Part 2 (NES) and Part 2/Junior (Game Boy) are the same size in pixels, even though the characters themselves are of different sizes.
What Could Have Been: Bubble Symphony has a Dummied Out witch boss, a giant female Dranko on a broomstick, who doesn't appear in the final game for whatever reason. Then again, a Dranko does appear in one of the bad ending cutscenes. Also, the dumped prototype shows the game was going to use a rendition of the original Bubble Bobble theme as main theme, which got used as a "secret" theme for the Classic Mode cheat in the final version.