The way I suspect some of them were made: get 10 random people, present the problem to them and ask each person what they think the solution is. Say no to the first 9, then say yes to whatever the 10th person guesses. If they guess something previously guessed, then keep prompting for more information until the solution is so specific even people on the right track will be confused by it.

Also add endless segments where several specific squares of the grid have mandatory items, something prevents you from systematically searching the entire grid, and if you go too far, you die.