Wikipedia has a good example: https://en.wikipedia.org/wiki/One-time_pad

In their example, "HELLO" is the plain text, "XMCKL" is the key, and the ciphertext is "EQNVZ". However, with a one time pad, an equally plausible plain text is "later" with the key "TQURI". Thus, without anymore data, it is simply impossible to know what the original message is.

Reasonable puzzles can be worked out (albeit maybe with a lot of work) with information provided by the puzzle or available somewhere in the environment.

Unreasonable puzzles (like some old Sierra games cough) are impossible without secret inside knowledge by the puzzle maker and/or brute force. And sometimes not even with brute force.

The hash/video example might just be an Easter egg hunt requiring looking across a wide set of videos (somewhat reasonable but boring), or completely unreasonable depending on circumstances.