Moving the Bar

Moving The Goalposts (Raising The Bar, Argument By Demanding Impossible Perfection)

If your opponent successfully addresses some point, then say he must also address some further point. If you can make these points more and more difficult (or diverse) then eventually your opponent must fail. If nothing else, you will eventually find a subject that your opponent isn't up on.

This is related to Argument By Question. Asking questions is easy: it's answering them that's hard.

It is also possible to lower the bar, reducing the burden on an argument. For example, a person who takes Vitamin C might claim that it prevents colds. When they do get a cold, then they move the goalposts, by saying that the cold would have been much worse if not for the Vitamin C.