These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Goddamned Bats: The blue spinning mooks in 2, which spray shots in all directions if not destroyed quickly.
3 has the small blue-and-yellow robot enemies, which pop up near the bottom of the screen via the sides and can collide with you if you're not expecting them. Then they fire at you from a very close range.