I would say anything from something minor to a parents divorce. I honestly think children need a stable family to grow up in, not one that yells and argues about everything under the sun. I look at it like you can train a Pitbull to be a fighter or the best loving family dog around. Children are the same concept its all about how they are treated and raised on how they will turn out. Thats just my belief though.
In terms of body development, poor nutrition and a sedentary lifestyle are the number one problems. In terms of cognitive development, children need one on one interaction to learn. Things like TV and tapes don't register before the age of 3 and so they don't actually learn... you might as well put a kaleidoscope recording on there. So instead of putting on Baby Einstein you should read and play with the kid (this is from the American Association of Pediatrics.) No matter the age, 1 on 1 is more effective than TV anyways. In terms of speech development, your child NEEDS to babble. Children who are not allowed to babble at a young age are the ones more likely to develop speech impediments or simply be late talkers. So when your child is 2 or 3 and making loud noises in the store, don't shush them - encourage them, they're learning!