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.
YMMV: Alien Syndrome
Demonic Spiders: In the NES version, the multi-sectioned worms in level 4 were replaced by purple mushroom cloud like things. They moved at least as fast as the player, with their shots traveling even quicker. There are ways to evade and kill them without dying, but it is extremely difficult.
Inferred Holocaust: The Alien Ship crashes into the earth.......... Plus, in many versions (especially the arcade), you could leave some, or even quite a few hostages behind on each ship.
Scrappy Mechanic: The short timer in the arcade. The maximum that the Dip Switch would even allow was 150 seconds. Which isn't much when you need to search a big level for hostages, then go fight a boss.
That One Boss: The level 5 boss, if it's the same on that showed up in the arcade.