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.
Demonic Spiders: The Pulsars, which appear starting from level 17 in the original. Each time they change to a new lane, they will shoot an electrical charge that will instantly kill you if you happen to be there while they release the charge. Even the control panel lampshades this by saying "VERY DANGEROUS".
In TxK, Pulsars lose some speed, but now their electrical charges last much larger and if one stays near the top of the play field too long, it will generate a Fuseball that will never leave the top of the play field until you kill it.
Scrappy Mechanic: In TxK, you can be killed by an enemy's bullet even if you're in a lane adjacent to where the bullet was fired.
TxK's Autosave system is also faulty. If you quit a game mid-progress by pressing Start then Select, it does not record the highest level you reached, along with the best score and number of lives you had for each level. Only by getting a Game Over with a new high score (in the local Top 10) will the game actually save.
What Do You Mean, It Wasn't Made on Drugs?: Developed by Jeff Minter, the sequels to Tempest constantly flood the game field with psychedelic visuals like flashes of light, particle effects, multicolored abstract backgrounds and deranged sounds. The flashiness had gotten so extreme in Space Giraffe that gamers protested that it made it almost impossible to understand what was going on in the level, so Jeff Minter made an alternate mode (called "NUXX", as opposed to the normal "Acid Mix" mode) where the backgrounds are dark.