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.
The small spider creatures that latch onto Duke (some even fall from the ceiling) and drain his health until you shake them off
The slug-like creatures that throw their eyeballs at you, then explode into a shower of eyeballs when you kill them
The ships that fly overhead and drop bombs with a rather huge blast radius; if you don't have anything besides the default weapon you're practically guaranteed to get hit (and even if you shoot the bomb as it's falling, it blows up anyway and the explosion will probably travel downwards far enough to still hit you)
The panther-like creatures made of green slime
The small ships in the first level of episode 2 which don't appear until you're practically on top of them