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: Dead Nation
Demonic Spiders: Cutters. They appear mixed in with swarms of regular zombies and can take tons of punishment, up to requiring an entire clip of direct hits with the rocket launcher on Undead to kill. To make matter worse, they can one-shot you on Grim if you took any prior damage if you have good armour and will one-shot you on Morbid and Undead, regardless of health and armour. Like the Jumper, they also ignore any potential distractions, instead focusing on rushing you and cutting you to bits.
Goddamn Bats: The trash mobs summoned by Mouths. They can be killed by any hit, including a completely unupgraded SMG shot. However, they don't give any money or multiplier orbs and come in swarms.
Good Bad Bugs: The Playstation 3 version has one where if you reload and switch to another weapon before the reload animation is complete, the weapon will be reloaded instantly. Fixed in the Vita edition, however.
The Vita has its own instant reload in the form of dashing. If you dash while the reload animation is going on, the weapon will immediately be reloaded once the dash begins.