Returned from Versailles the other day, and while I had just purchased a couple regular ticket for there and back, another family had to learn the hard way when they purchased their day passes. To my knowledge, they purchased a 1-day travel pass for the RER so they could do Versailles, and then afterwards back in Paris, see more sights. Well, they thought that upon purchasing that was validation, and did not see that on the little ticket pass itself, there is a spot to put your name, the date, etc. When ticket collectors came around to check tickets, they checked the group and realized that they had not filled out the card (a.k.a. validated it) and were charged a fine. Apparently, it's supposed to be 50 Euros, but they only charged 30 something.
So just a word of warning I thought I'd give for those that don't notice the little form section on the day pass ticket - validate it before, because they won't tolerate you filling it out upon discovering you've not done it at the inopportune moment.