I have some answer to this question but it is protected. Can you add a comment there (I hope this answer can help someone) before downvoting or deleting it - thanks:

Example is blackbox password function f(x). You can try f for many parameters and it always returns 0. You consider it as a proof that this f will always return 0. But in fact it returns 1 for correct password as parameter and 0 for anything else.

• This is a question for meta. – Angina Seng Sep 26 '18 at 6:42
• I did it for you but may I suggest you earn $10$ reputation points to do it yourself in the future? Then you can participate in the conversation yourself without need of a mediator and hopefully avoid communication errors? And note- you can post comments on your question. – For the love of maths Sep 26 '18 at 6:46
• Why would you work around a moderator's decision ? – user65203 Sep 26 '18 at 6:56
• @YvesDaoust assuming that this is directed towards me, "This question is protected to prevent "thanks!", "me too!", or spam answers by new users. To answer it, you must have earned at least 10 reputation on this site (the association bonus does not count)." As this is seemingly not a spam comment, I posted it, despite not understanding it complete. If it goes against any site policy, I am perfectly willing to remove the comment. I apologise if I made a mistake and would like to thank you for pointing it out. – For the love of maths Sep 26 '18 at 7:23
• @MohammadZuhairKhan: no, I am adressing the OP. – user65203 Sep 26 '18 at 7:30
• I know it's kind of dilemma to get all good comments and not to get any spam comments thus resulting to some kind of protection rules but I don't want to build reputation. As the time will advance, all internet sites will have more and more issues with junk contributions and even lazier contributors. I hope you'll find some solution for that. – Jan Sep 26 '18 at 7:40
• The solution that Stack Exchange uses to avoid "junk contributions" is to require posters to prove that they can contribute positively to the site before allowing them to answer certain questions. – Joel Reyes Noche Sep 26 '18 at 12:15
• @Jan Honestly, question protection is also meant to prevent low-quality contributions from users who aren't yet familiar with MSE and what is expected in an answer. The answer that you've included here, unfortunately, is exactly the sort of thing we're trying to prevent. It's short and vague with no real explanation, and it's mathematically incorrect; "getting 0 a bunch of times" does not constitute a proof that the output is always zero, so we cannot "consider it as a proof." – user296602 Sep 26 '18 at 13:45
• @T. Bongers That is exactly what is solved in the question itself - what is proof and what is not. I just wanted to include the most trivial example function which, if blackboxed, doesn't provide you any clue that there is any other value than 0. My contribution has value in the context of original question. Outside of it contribution looks non-precise, fuzzy, not understandable, etc. There are other consequences from other answers, e.g. large but finite possible parameters space. But I don't want to repeat others. It had to be a comment to consider and not THE complete answer anyway. – Jan Sep 26 '18 at 14:27

Different amounts of reputation are required for these actions. Since the Question is "protected", one needs to have earned $$10$$ points for the privilege of posting an Answer there. This mechanism of protected Questions is meant to limit the number of low-quality Answers that must go through a review process.
On the other hand you perhaps wanted to have the observation posted as a Comment. It is true that Comments in the StackExchange model are considered ephemeral content, so that completeness is not expected. However $$50$$ points are required for the privilege of commenting on the posts of others (you can always comment on your own Questions and Answers, and on the posts of others that answer your Questions).