Things you don't say to a parent...EVER.

1. You should never tell a parent how to parent. I don't care how many kids you have, what kind of experience you have, how many kids you used to babysit, what kind of books you read, etc. Don't tell me how to be a parent. (Granted, there ARE exceptions to this rule, but - generally speaking - if you are a relatively good parent then you don't need anyone telling you how to be a parent...ya dig?)
2. You should never ask a parent if their child is ADD/ADHD just because they are "bouncing off the walls." You can either leave, ignore the behavior or just get over it.
3. It's not nice to accuse a parent of being "unfit" when the only reason the parent is unfit is because he or she just doesn't exercise. :)
4. Don't ever, ever, ever tell a parent that their child resembles a murdered child you saw on the news. I mean, really, how insensitive can you get?
   
5. When congratulating a new mother (or a new father for that matter), don't immediately question her age (or his age) and her motives (or his motives) for having a child. Seriously, people, your sincerity factor goes right out the window.
   
6. Never say 'well back when you were a kid we did it this way'. Yeah-back in the you also used lead paint and gave kids aspirin...
   
7. Never ever say 'my kids would never do such a thing.' Chances are MY kids learned it from YOUR kids.
   
8. Never say 'are you going to let your kid get away with that?' Yep sure am.

Ugh. I'm stuck at 5 things...I know there is more. I just can't think clearly today. I've had this list generated for quite some time, too.
Just for the record, these are things that have either been said to me, I've had to deal with or things I've just heard in general. People are flat out mean sometimes.
UPDATED!! Items 6, 7, and 8 have been added courtesy of my friend, Rebecca. :)