@flo Same thing.
There are two ways a sudoku problem can go wrong.
1. There may not be any solution. E.g. if the guy who messes up puts two “3’s” in the same row.
It can be more subtle though. E.g. you start solving the problem and you say “ahh, this cell needs to have the number 7.” Then you keep going and realize that a 7 in that cell conflicts with something else. As long as you didn’t make any mistake, this is a messed up puzzle (or as you put it, a number placed in the wrong box).
2. There might be more than one solution. E.g. a totally blank puzzle. There are a huge number of solutions. But it doesn’t have to be this simple. For example, if the problem you are given has neither a 3 or 5 in it, as long as you don’t run into problem 1, there are at least two solutions. (If you find one, you can find another by swapping all the 3’s for 5’s and vice-versa).
If this happens, there is a solution but it isn’t “unique.”
You may or may not consider this to be a “bad” puzzle…
A sudoku solver is a program where you enter the problem (i.e. type in the numbers you are given) and hit “go.” It then runs through every possible combination until it finds a solution. Some of them stop once they find a solution. Some of them keep going and find all the solutions. If you can find one that does this and just tells you how many solutions there are (but doesn’t display them) then you have a solution to your issue. I’m sure there are some other there… its something anyone with a few programming classes under their belt can probably pull off.