Revealing mistake: When the TVs explode, the glass is very fake and "cardboardy".
Revealing mistake: Madison is wearing jeans as she walks towards the group of tourists although she's supposed to be naked.
Revealing mistake: When Allen is in the park and drives backwards to avoid the MPs, there's a closeup where the pigeons fly backwards, revealing the angle is the previous one with the birds taking off but played backwards.
Revealing mistake: When Hanks drives through the city escaping form the MPs, the guy driving is a very obvious stuntman.
Chosen answer: Whilst it may be a tad unusual, each system serves as a back-up for the other.
David Mercier