Continuing the general theme of limitation, here’s a variant that I have devised where a blank cell plays the role of a digit, to be placed so that there is exactly one blank cell in each row, column and 3×3 block. What makes the rule amusing is that you cannot distinguish blank cells that have to be filled in with 8 of the digits and the special blank cells representing the ninth digit. I’ve not seen it in any other Sudoku variant sites, so you may be seeing it here first.
It’s abtractly identical to many of the Sudoku variants where other symbols are mapped to the digits 1-9. In this case, only one digit is mapped to blank, while the rest of the digits stay as they are.