I have been an avid Sudoku fan for about five years, but in all that time I have not been sure what constitutes an invalid puzzle. I have been reading a lot on the subject but I am still not sure. I hope someone here can straighten me out.

Here is an illustration I got from the Hodoku site. The simple sample below shows a rectangle with three givens and one not given.

2 Given 4 Given

4 Given 2 NOT Given

According to Hodoku the 2 on bottom must be removed to avoid an invalid puzzle. However, a great many times the puzzle does not permit you to eliminate the 2, in other words the 2 on the bottom become the candidate for that cell. Does that prove that the puzzle is invalid?

Your help would be appreciated.