Whenever you have a system that relies on probability (eg the extremely low probability that an image might match two independent perceptual hashes) you have to

Whenever you have a system that relies on probability (eg the extremely low probability that an image might match two independent perceptual hashes) you have to take two things in to account; 1. There is a possibility, however tiny, that an innocent image will still match both hashes. The probability is not zero. It can't be. They're hashes. 2. By telling people that there is no chance of an innocent image matching both hashes you are forcing the burden of proof on to the victim. If someone is unfortunate enough to have an image that matches both hashes they will be dragged into a law enforcement office and told to explain "why the foolproof, perfect, impossible-to-cheat system said there was child porn on their phone". It will be on them to explain why the system isn't correct. The presumption of innocence is lost when too much faith is placed on technology. That is why this is dangerous. Arguably it's a well-designed system that safeguards children and catches despicable criminals, but unless people understand it isn't infallible, and stop arguing that it is, then it could cost an innocent person their freedom. That's a high price to pay.