Here's what the Lexicon has to say on the potion:

Veritaserum:

The most powerful Truth Serum available, this colourless, odourless potion (which looks like plain water) forces the drinker to tell the truth.

A Truth Potion so powerful that three drops would have the drinker spilling his or her innermost secrets.

Use of this potion is controlled by very strict Ministry guidelines.

(...)

An alert wizard who's good at Potions can protect himself against the use of Veritaserum by keeping some antidote handy.

I was wondering if this potion was entirely accurate - is it possible for someone to protect their secrets by believing something actually happened?

For example, if Luna believed that Harry was at dinner, but he was in the Common Room, would she tell the truth or the truth as she sees it if she was given Veritaserum?

Thanks,
Cammie!