I've gotten similar cards to this (or maybe the same one) before. This one is from Liz, the other submitter who I forget her name got it form a mother-in-law. It must be like a passive aggressive way of saying, "just because you married my son doesn't make you REAL family."
I don't usually post the following: newspaper headlines, personal email, craigslist postings, unprofessional websites. I also tend to not crosspost things from other blogs, since I have so much unique material waiting for me to get to it.
Things I see a lot: silica gel "do not eat"; hair dryer labels; inside the bus "do not drill"; Wal-mart sign about IDs; coffee machine with "2" cup sizes; employees must "wash hands".