on LA Noire what happens when you accuse someone of lying when in fact they are telling the truth?

Answer 1

You have to choose on the list of clues on which you are accusing them of. if you falsely accuse them, you get the answer wrong and will earn less points when you level up.

Answer 2

It's true that you won't get as many experience points. Furthermore, you might miss out on some information that would be helpful for successfully completing the case. For more information on getting the questions right, see these guides: http://www.sleeandtopher.com/gaming/guides/la-noire-case-guides/