Existential threat A catch-all sign for things you really don’t want to mess with – existential threat implies threats to the future of humanity as a whole.
Cognitive hazard Exactly what kinds of hazards could occur with mature cognotechnologies is hard to imagine. This sign represents a general hazard, perhaps the induction of inconsistent beliefs, infinite loops or mistaken perception.
Chaotic system Chaos control is likely to be very useful in many future applications. But chaos is sensitive, so interfering with such a system might be unadvisable.
Group intellect Don’t go too close, or resistance will be futile… While well-behaved group minds no doubt are selective of who joins (it is after all rather intimate) and unlikely to assimilate everybody nearby, there might be applications or situations where mental firewalls are down and brains easily form group intellects. Maybe the people in the icon should all be raising their hands in the same way, but this is the clipart I found.
Ubiquitious surveillance Using microdevices, identity technology and pattern recognition software it is not inconceivable that future environments will be 100% privacy-free. If this is not the standard situation but only occurs in some places, this sign would mark the surveilled area. In the case of total surveillance the opposite sign, with a crossed over or blindfolded pyramid, would mark the risky non-observed areas.
Self-evolving system A system that evolves freely is potentially very adaptable and creative. It could also become nearly anything, with consequences ranging from the annoying to the disastrous. It is likely unlimited self-evolution will need to be contained carefully even as we mine it for truly new inventions. The arrows nicely hint at a chaos-star as well as replication.
Memetic hazard A black lightbulb to represent really bad ideas.
Macroscale quantum system Macroscale quantum systems are likely not personally dangerous, but interfering with them will of course disrupt whatever they are doing. Of course, just seeing the warning might be enough to decohere them. Some might interpret the psi sign as a warning for psychic powers. Maybe the sign could show an alive/dead cat instead, but it is a bit too much of an in-joke.