Nowadays there are warning labels on most everything: this is flammable, that will cause injury because it's sharp, this substance is poisonous, and that canister is under pressure, but what types of warning labels would you need for items designed to be used in a weightless environment?
There is a very large set of possibilities. For real-world examples do some research on what NASA, Roscosmos, JAXA, ESA, CNSA do. The list below is just off the top of my head.
Cannot be used in microgravity.
Must be used in gravity field greater than X% of earth.
Must be used in gravity field less than X% of earth.
Cannot withstand acceleration greater than Y.
Must be aligned along a certain axis for acceleration.
Must be used at an atmospheric pressure greater than X.
Must be used at an atmospheric pressure less than X.
Cannot be used in Heliox atmosphere.
Must only be unwrapped/used in an environment with a scrubber that can handle a particular chemical.
Must be decontaminated via (some method) when moving from atmospheric composition X to composition Y.
Must be exposed to hard vacuum for X hours before being brought into atmosphere.
Electrical/Magnetic/Cosmic Ray Environment
Requires shielding in a certain EM range.
Requires shielding from certain cosmic rays.
Cannot be used during a solar flare of greater than X intensity.
Cannot be used if organism X is present.
Can only be used if organism X is present.
Cannot be taken to an uncontaminated planetary environment.
Can be injected into a person only if they have a certain symbiote/implant/genetic marker.
Can only be used in temperature range X to Y.
Can only be stored in temperature range X to Y.
Do not change temperature at a rate greater than Z/sec.
The already existing answer is great, but besides the sciency bits, a lot of the warnings will probably be the same, just more important, as outer space is significantly less hospitable than Earth's surface.
Fire warnings, for example, will still be very important, as a fire within a spaceship would be catastrophic. In fact, flammable materials should be very rare in a spaceship for that very reason.
As for things that are unique to outer space, there are plenty of dangers to warn about in a spaceship. Glaring red warnings on airlock doors, and above anything that might affect the life-support systems. The air, water and temperature systems are essential, so anything that might affect the power supply should be fraught with warning labels.
Hazards in zero gravity... you need warnings about whether or not something attached to a wall can easily come detached if you tug on it, so you don't accidentally fling it across the room, or so you don't use something unsuitable for a handhold.
You need to be particularly careful about spilling liquids in zero gravity, as they'd be particularly hard to clean up.
For accelerations and decelerations, there will be all manner of fancy seat belts, and little decals on the walls showing cartooned explanations of how to put them on. Think of the safety cards you see in airplanes- you'll want an equivalent of those, too.
Many Earth products have ridiculous labels, like "don't put anything living in this microwave oven", "Caution: the coffee in this cup can be hot", "do not point loaded guns directly at your face", "warning: this ice cream melts in the sun", labels that sound stupid to put on objects because they seem so obvious. However, each of these labels has been put on there because someone, somewhere, at some point, did exactly what the label said not to do, was injured or lost the product this warning pertained to, sued the company and won.
I see no reason why space products would be any differently. "Warning: do not fly this space ship too close to celestial objects". "Warning, do not stick any appendages in the automatic food generator while it is active." stuff like that.
The thing about silly warnings on earth is that most of them were added because someone, somewhere actually did it and did try to sue the manufacturer. Whether the warning can be followed in practice is irrelevant, as the only purpose is to try to give the manufacturer more leverage in case of a legal claim.
So considering the actual incidents and accidents that have occurred in spaceflight, we can color things up a bit and warn:
- On fecal disposal bags: "Ensure bag is securely closed" (Apollo 10)
- On hotplates: "Do not use in oxygen atmosphere" (Valentin Bondarenko's death)
- In ocean landing capsule: "Astronauts need adult supervision while in water." (Sergei Vozovikov's drowning)
- On spaceplane braking system: "Observe caution when deploying while vehicle is in motion." (SpaceShipTwo)
- In ocean landing capsule: "Do not use capsule if door seal is damaged." (Libery Bell 7)
- In airlock: "Manufacturer is not responsible for difficulties in re-entering the spacecraft that are caused by physical size of the astronaut." (Voskhod 2)
- On fuel oxidizer canister: "Hazardous - Do not breathe!" (Apollo-Soyuz Test Project)
- On exercise equipment: "Use eye protection!" (Norman E. Thagard eye injury)
- On space suit: "Discontinue use if you feel like drowning." (ISS Expedition 36)
Not for use in artificial gravity.
The artificial gravity systems that exist on spacecraft sometimes have weird effects on items. Symptoms include:
- Repulsion instead of attraction
- Wrong gravity factor
- Random Annihilation of atoms in item
- Sporadic nuclear decay
- Possible corruption of the computer running the universe
"CAUTION: Small carcinogenic particulate matter; do not inhale; open only in gravity or well-ventilated area. In case of accidental release into atmosphere evacuate room and call your Life Support Administrator."
As far as typical consumer goods go (i.e., not guns, don't generate EMP, not infectious, etc) the relevant difference about space is that you're in microgravity a lot of the time. And as ISS astronauts have told us, keeping things neat in microgravity is tough; dust and liquids will often just float about indefinitely. This gives them a much better opportunity to get inside of you and cause trouble. So, while on Earth a "do not inhale" warning on a, say, box of dishwasher soap likely isn't necessary, in space it would be a necessary legal disclaimer.
There are some collections of such signs on the net already. Here is a short (comical) selection, and here and in the linked Flickr page is a huge collection of warnings signs for the world of tomorrow, though not exclusively for space.
So depending on how serious and futuristic you want the signs to be, here are some candidates:
- autonomous vehicles in operation (make sure you are recognizable as human)
- malicious autonomous vehicles in operation (enter at own risk)
- nanobot contamination zone (life-long quarantine required if they touch you)
- AI experimentation zone (don't give them any ideas, or any unsupervised input at all)
- holodeck ahead (anything you see here may be an illusion, including any exit signs)
- antimatter (don't interfere with the containment fields)
- very-high-density matter (read the "Neutron bullet" chapter from What-If book for details)
- steep gravitic gradients (gravity has different orientation and force all over the place, so watch the floor markings and keep vomit bags ready)
- distorted space-time (stay away from anyone who looks like your grandpa)
- area without network/internet connectivity
- serious version: if you have an accident in this area, no one can hear you scream, so have someone else watch you
- non-serious version: staying in here may cause feelings of loneliness
- other non-serious version: staying in here may cause your brain implant to become unsynchronized with the rest of the hive mind (or maybe it just misses the latest security updates and you get a malware uploaded to your brain next time you go online)
- alien overlord area (if they notice you they'll wipe you away like the bacteria you are)