I just bought a brick of Bicycles from 
www.cards4magic.co.uk and they are very obviously seconds. And, no, they weren't sold as such!
These cards are not just off by a millimeter or so. Oh, no. These cards are off both horizontally AND vertically, and the white border at the top is TWICE the border at the bottom.
Not pleased.
Maybe it's not their fault? It's not like they can open every box and check. Even so, I'm going to buy from elsewhere next time, just in case.
Anyhow, I just mention this in response to jokerdan's comments about using factory seconds. If the factory seconds are off both long edge and short edge, or if they're off by, say, 4mm (I haven't actually measured), the cards won't be much good even for this purpose.