A good grid should only have one correct solution with the links that the setter has selected. Note that this doesn't prevent “red-herring” clues that can fall into multiple categories. The important thing is that despite these red-herrings, there is still just one full solution. Something to try to avoid is a situation where two clues can be swapped. For example, if group A is “things that can go on a pizza” and group B is “fruit” then it's fine to have “tomato” in either group. But if you put “tomato” in group A and “pineapple” in group B, then there is another solution equally valid with pineapple in group A and tomato in group B, which makes the grid problematic.