The game contains an unused ability, "Block". It appears to be similar to the Triple Star ability, and is entirely functional beyond being intentionally disabled by making Kirby immediately lose the ability.
In stage 8-3, the slope you encounter in the second room in the Japanese version is a little glitchy. If the player decides to walk and subsequent crouch on the slope, this could possibly put you behind the slope into the out-of-bounds area. This slope happens to be the only occurrence in the game to do this. Crouch-sliding into a wall can make Kirby partially enter said wall. This glitch was fixed in the International version by making the slope completely solid so you couldn't interact into the out-of-bounds area.
In the overworld map of World 7, a shadowy figure can be seen, once the player reaches the area, it is clear that the figure is the Halberd, Meta Knight's airship. It is thought to be the same Halberd that crashed into the ocean at the end of Meta Knight's Revenge in Kirby Super Star.
In the original Japanese version of Kirby: Squeak Squad, Spinni is referred to as female, but no mention of her gender is made in the game's English version, possibly to avoid any controversy with gender issues. However, the American version of Kirby Mass Attack refers to Spinni as male.