Unless the poster of the question was lying then there is some sort of glitch (at least to that one specific game, event, time or place) that caused the Master Ball to fail. Now it's completely possible that the asker heard the story and indirectly asked if it was true, or probably just remembered the event wrong and used some other type of Pokeball. Meaning you're probably right. Plus @ Indigo 'catch rate' is just a bunch of code affected by modifiers such as target health, status, pokeball type etc etc, with the masterball just setting this catch rate to 100. Unless there was some conflicting code (which would probably cause far more errors than just the ball failing) or Giratina/some other variable has some hidden code purely for it that means it's catch rate can't be 100%, which is very unlikely, it should work.
In any case however, this answer should have a detailed explanation why and some evidence if possible. Go through some possibilities, explain the code - whatever. It's still an actual answer and should be treated similarly to all others. I.e put some effort into it.
One more thing, if you think the question/answer breaks rules then flag it. An Editor/Moderator will look over it and decide.