by ab » Sun Mar 30, 2008 7:55 pm
there are two reasons I can think of why you're having a problem when you make eliminations using an x-wing. One is that you made a mistake and it turned out there were 3 or more of the specific candidate on one or both of the lines. The other is that the puzzle that you were trying to solve doesn't have a solution. I guess there is a third possibility, that you haven't quite understood how to search for/apply the x-wing technique and you're making eliminations that don't follow from the x-wing.
As solgor100 said if you are applying any of the standard techniques to a valid sudoku puzzle, they will not cause invalid eliminations, period.