More pics for Baker.
Entire races of aliens exist just to eat the occasion human being that might pop by every century or so. Their physiology will be exactly tailored to take advantage of this rare source of nutrition. They must also use it in their breeding cycle.
Personal body armour is entirely useless, deflecting neither sword blows or laser bolts - in fact it has never been observed to prevent a single injury of any nature. Evil Empires will nevertheless spend fortunes decking out their Legions of Terror in these outfits because they look cool. Or something.
On planets that have regressed to medieval or earlier historical conditions, males are best armoured by covering as much of the body as practical, except for the arms which must be bare. Females only need armour on their breasts and loins, the rest of the body is best protected by naked skin.
No weapon, whether personal sidearm or fearsome battleship turret, can benefit from any form of targetting software.
Exploratory missions into unknown areas of space never ever come across a dangerous form of energy, or a deadly alien disease, which has been encountered previously and everyone knows precisely how to deal with. Despite this there is always some trivial modification of a handy resource which just … might … work, and in fact does.
The known universe’s archival facilities are in such a poor state that entire populated planets - nay, solar systems - are routinely forgotten, despite (judging from the familiarity of language and social systems) having only been sidelined for a century or two.
Isolated colonies consisting of as few as two members, usually father and daughter, are nevertheless completely viable and proof against all planetary dangers. However the arrival of a human-crewed spaceship invariably signals the end of the colony, as the father is doomed to die within days and the daughter will fall in love with the ranking officer, despite never having met a human being she was not related to before.
Once the atmospheric dome on a planet, or air-seal on a spaceship, has been punctured it takes at least half an hour of movie time for oxygen levels to drop to dangerous levels. Conversely simply blocking the puncture, or at most restarting the pumps, is enough to restore O2 levels immediately.
Avoid all eye contact. If there are no eyes, avoid all contact.
They’re in everybody’s eggs.
Once you eat the blue moss, you become one of them.
All the aliens play the piano beautifully.
If you kill one of them, don’t take its weapon. That way your own weapon will run out of ammo two pages later.
Some aliens will take over by impersonating us.
God bless Slim Whitman.
Space aliens believe that Bugs Bunny screws his mother, Abraham Lincoln is dead, and that the Cubs suck.
Mars
needs
women.
Everything you know is wrong.
In the next world, you’re on your own.
Monsters from the 1950s love nothing better tan carrying a woman off in their arms.
Since the 1970s*, aliens simply cannot reproduce without a human being intio which to lay eggs or their larval form. God knows how they did it before they found people.
Always look around for any overlooked or forgotten its of Monster Tissue, because too damned many of them can regenerate from teeny bits.
In particular, Hands can crawl around on their own and, even without sensory organs, find victims**. Even without leverage, they can throttle you. Even a wounded hand can take you in an arm-wrestling match.
You only get One Chance to kill the monster with your cobbled-together weapon, or there’s only enough Magic Monster-Killer Stuff. Fortunately, no one in the history of monster flicks has ever botched this One Shot.
Explosions always last five minutes. If you have to, you can outrun the fireball.
*Except for that ahead-of-its-time 1950s MST3K-fodder Corman clunker Attack of the Blood Beast.
** Exception – in Attack of the Saucer Men the hands easch have their own eyeballs. And can ij=nject you with ethanol!
Ship computers will respond to any voice command. For example:
Guy: Computer!
Ship: makes bleepy recognition noise
Guy: Could you tell me…nah, forget it.
The computer will always understand exactly what you mean. It will not even attempt to tell you the definition of “nah, forget it” or what language you said it in. It won’t ask you for clarification. Other terms the computer will understand:
“Never mind”
“Whoa”
“Peace out”
“Later, Gator”
Does not compute, please rephrase in Loglin or Esperanto.
Banana2000
Someone picked up a copy of H. Beam Piper’s The Complete Paratime, didn’t they? 
I’d bet you’d be hungry too, what with all that inventing and scheming and flying and breaking the lightspeed barrier and such.
UFO’s are big rigs, they come from outer space
Stoppin’ off at the truckstop Earth
Lookin’ fer a place ta eat.
{Mojo Nixon, UFO’s Are Big Rigs}
There are loopholes in the laws of physics.
The definition of the very word UNIVERSE must be wrong, since there are many "all there is"es.
Singing mailboxes means something.
Clearly you haven’t seen Galaxy Quest and Lt. Tawny Madison, whose job it was to be the single point of communication for the ship’s computer: “Look! I have one job on this lousy ship, it’s stupid, but I’m gonna do it, okay?”
Stranger
Gilligan’s Island: those poor people!
That way her attacker is too busy trying to get a glimpse of nipple to notice the mortai heading straight into his solar plexus. The tinfoil underpants are protection against microwave weapons.
I had a cat I named Tribble. (She made the same sounds.)
Sadly, that ended badly as well.
Got into the poisoned wheat, eh?
Every major advance in any field in the 18th thru 23rd centuries was a Russian inwention.