SP, I like the Blue Peter suggestion. "Sorry, you cut outside the lines..........no flight for you!!!"
Personally, I think it's going to cause a LOAD of hassle, especially when people print off the wrong page, turn up with a piece of paper that looks like it has been eaten by the dog (just like the old homework excuse!).................but they're going to demand to be let on the aircraft with what they've got!
Also, errors in boarding cards occur. In Brisbane last month, both boarding cards we were issued were for the same seat. I only happened to notice when I glanced at them when I showed them at the duty free!!! It's going to cause problems.