I can go either way on this. I see it as a formality. I do take to heart the intent. In other words, I intend to tell the truth, the whole truth, or whatever the circumstances are.

I wonder if anyone's fake oath has been challenged? For instance, could someone claim that a witnesses testimony could not be relied upon, once it was found out that he/she has did the "so help me god" thing, despite being an atheist.

The strongest, or most militant don’t. The First Family of American Atheism had members spend time in jail due to their unwillingness to ‘so help them god’ in order to serve on a jury. They were further harassed by the courts for continuing their refusal. Many do not allow their sons to join ‘the boy scouts’ due to their god oath..

It all depends on how far you care to take it after resisting. At least there are organizations (in the USA) willing to defend the most egregious of these demands. As has been described around here before, many government agencies now give you an opportunity to ‘affirm’ without the need of a ‘god.’ But - those rights were only obtained by those willing to fight for them!