Being a slave to your baby monitor
If your baby cries when you put her down at night and you rush in to comfort her each time, she'll never learn to soothe herself to sleep. (Of course, if your child is sick or in distress, you need to help her.) Turn down the monitor so that you don't hear every whine and whimper; when she really cries, wait it out for 5 minutes before you go in to check on her. Tomorrow night, extend the check-in interval to 10 minutes, and so on.