Secretly break the rules: Breaking the rules set by Mom and Dad undermines their authority in front of the grandchildren, so if they enforce a specific bedtime, stick to it.
Expect kids to parent the same way: "Back in your day" was good . . . for back in your day. Times change, parenting evolves, and good grandparents change with it!
Compare grandkids to one another: While one of your grandchildren may be a better athlete or student than another, you don't need to mention it — the odds are that they're already hyperaware of their differences.
Guilt new moms about their baby weight: Losing weight after having a child is no picnic, and pressure from parents won't help at all. It's better just not to mention weight at all!
Take the grandkids for surprise haircuts:Haircuts are a big deal for little kids and should always be discussed with parents first — don't try to surprise Mom and Dad with a hip new bowl cut.
Pry the grandkids for information about their parents: Never make your grandchild the middleman during fights, holiday planning, or divorces. It puts unnecessary pressure on them and simply isn't a healthy way to handle familial relationships.