If you brag, or even really utter the words to anyone, that your baby is sleeping through the night, the next few nights will be a living hell, waking every two hours.
Same goes for potty training. Once you start telling people your kid is potty trained, prepare yourself for a crap fest, because the kid who never pooped in their pants ever will start, right then.
Your baby who always takes short naps, will decide to take nice beautiful long naps at the most inconvenient times. For example, you make an appointment at 3:00, because your baby is always awake by 3... guaranteed he/she'll be sound asleep that day and you'll have to wake him/her up to make it to your appointment on time.
The cutest outfits, the ones you spent the most money on, or treasure the most for whatever reason, will end up covered in poo. Somehow they sense these things.
The moment you close your eyes and kid yourself that you will get to take a nap today, your baby will cry. Even if they just fell asleep 10 minutes ago!
If all the kids are playing nicely and quietly, and the phone rings, do not answer it. It will completely end all sense of peace, the moment you try to have an adult conversation.
I have more, just can't think of them now. I'll have to add a part two one of these days.