I know there already is an answer, but since my mom does the Sudoku in the newspaper(s) on a daily basis, and she's afraid of spiders, I gave her this very fitting Spidoku to complete. ;p
Here is the result:
The problem is that the unique solution to the sudoku does not have a 7 there. At the moment it is not immediately clear what number should be in that corner, and there is no direct reason for it to be a 7 rather than a 2 or a 6.
In fact, with some thought you can show it will actually have to be a 6:
This program likely has the solution built into it, so it knows that this 7 is wrong.
Each Sudoku puzzle has a single solution that can be figured out with pure logic.¹ Even if you place something without it directly contradicting anything else, it may be the case that you would find out later that that was wrong. You should only place numbers that you've ...