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 a day ago | parent | next [-]

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 a day ago | parent [-]

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

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

	▲	shagie a day ago \| parent \| next [-]
		https://www.sudokuwiki.org/sudokux.aspx is a useful site. I'm personally a fan of the killer variety. Its solver is interesting in showing the individual steps. For the same puzzle, https://www.sudokuwiki.org/sudokux.aspx?bd=06039000804900150...
	▲	zem a day ago \| parent \| prev [-]
		if you like sudoku, you owe it to yourself to check out the "cracking the cryptic" youtube channel and get introduced to the delightful world of variant sudoku.

▲

EdSchouten a day ago | parent | prev [-]

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

	▲	rmunn a day ago \| parent [-]
		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.