That you should pick the baby up anytime they cry. And also that you should feed the baby anytime they cry. What the...?! As new parents, they told us that in the (attachment) parenting class we went to. The first year of parenthood was so miserable for us because literally every little whimper, I ran to the baby and tried to feed her. Of course she wasn't hungry all the time so it just made her more unhappy! That, and that if I "wore" the baby (in a sling) that she would be more secure. She hated the sling from day 1, and so did I because it made her so unhappy.
After calling my grandmother (the mother of 7) for advice, she told me to stop being an idiot and feed the baby at normal times of the day (the same times every day, give or take), that sometimes babies just need a good cry and that's okay, and that if I don't want someone clinging to me 24/7, probably my baby didn't want that, either. I followed her common-sense advice, and my baby was happier, I was happier, she ate and slept better... I guess grandma knows best! Ever since then, I don't care what the "conventional wisdom" is, I just do whatever works and is easiest!