Alien Syndrome/YMMV


 * 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.
 * The level 6 boss of the arcade version.
 * That One Level: Level 4 in the NES version, level 5 in most others.