[neogodless]

I'm 99% sure this was an invalid Sudoku puzzle.

Just before putting the 2 in here (above the pencil 6), I put in 6 and it said Mistake, so I erased it and put 2. But... why wouldn't 6 be valid there?

https://imgur.com/a/aOnKbiT

EDIT: As per replies, "X" Sudoku is variant with a different rule. While I saw the diagonals "highlighted" in another color, I didn't know that rule. Perhaps it could be added to the page for those unfamiliar with this non-standard Sudoku variant?

[doodpants]

Because there's already another 6 on the same main diagonal. This is an "X Sudoku" puzzle, which means that each main diagonal must have all 9 different numbers.

[neogodless]

All I've never done the diagonals before... oops.

https://imgur.com/a/Dfxf9CJ (before picking 2 / 6 / 7.)

[EdSchouten]

Yeah, that Sudoku puzzle has multiple valid solutions, whereas the page only seems to accept a single one.

[rmunn]

There's an extra constraint on that one: the two main diagonals must both have nine different digits. That's what makes it a single-solution puzzle.