Questioning Truth and Reality

I've had some issues with the Potions & Swearing by Magic... in canon, and most fanfiction as well, Veritaserum is less effective on master Occulomancers... meaning Snape and Dumbledore could sway, though not negate, the effects of the potion. The potion works on what the individual knows and believes to be true... here's an example:

Typical Wizengamout questioning of Hermione under the effects of Veritaserum -

Hermione is lead into the chamber and forced to sit in the uncomfortable chair in the middle of the floor, chains leaping to securely fasten her down.

Hermione - The bushy haired brunette looks about in confusion, "Why are you doing this? I've done nothing wrong."

Umbridge - "Silence. Auror, apply the Veritaserum." The toad like woman in a hideously pink cardigan says viciously, an overly wide grin stretching her face is disturbing ways.

Auror - "Don't fight it miss. It's easier that way."

Hermione glares at Umbrige but takes the Veritaserum filled water and drinks it swiftly. Her chocolate brown eyes dulling somewhat as the potion takes effect.

Fudge - The Minister for Magic nervously fingers his lime green bowler hat, "Let's test to see if the potion has taken effect. Ms. Granger, what is your full name?"

Hermione - "Hermione Jean Granger," the chained girl replies monotonously (yeah I've seen Jane a lot too but I prefer Jean and it's not in the books or movies. Albus' Will has it as Jean as well if you are looking at harrypotter-wikia as you can read it that way on one of the movie props.)

Umbridge - "Hem, Hem..." the toad-like women says to draw attention to herself, "Are you a Wizard?"

Hermione - "No, I'm a Witch." (This is a point of contention... the answer is 'No,' which fits with the Umbridge narrative that muggleborns steal magic, but it is not stated in canon that she can't expound to add that '[she]'s a Witch' which changes the question from 'are you a mage?' to 'are you male?')

Umbridge glares at Hermione for not answering with a simple 'no' the way she was supposed to.

Amelia - The Head of the Department of Magical Law Enforcement cocked her monocle bearing eyebrow at her colleague's demeanor, "One last question before we begin. Is the sky blue?"

Hermione - "No. It's transparent. You see, light is refracted off of..."

Fudge - The Minister explodes out of his seat shouting angrily, "WHAT THE HELL ARE YOU TALKING ABOUT? THE SKY IS BLUE! ALL YOU HAVE TO DO IS GO OUTSIDE AND LOOK UP!" (Hermione is about talking science while Fudge is talking about appearances... both are 'true' but it's a difference of perspective on the question).

Umbridge - "We obviously can't believe anything this girl says. Even under Veritaserum she's lying to us. We should give her to the Dementors."

Fudge - "Seditious Muggle non-sense... I agree."

Amelia is shocked by the blatant disregard of the Law. Shaking her head, she discreetly signals the Auror beside Hermione to take the girl to one of the department's secure safe-houses. When those who determine the law break it legally, those who enforce it will have to find new methods to help those who are innocent.

When swearing on their magic... they can not lie... but carefully worded answers to mislead the listener are perfectly acceptable, which is very much in keeping with both canon and fanon regarding Dumbledore and Snape. An Obliviated memory is no longer there so if a person isn't telling the truth, they 'don't know' they are lying and the Veritaserum has nothing to trigger on.