Man, SIDS. It's specifically non-specific, but the worry it causes is quite specific.

My daughter, as a baby, always managed to find a way to sleep on her stomach. Wouldn't sleep on her back, but almost magically by comparison would fall asleep lying on her stomach (face to one side or the other, not straight down, obviously - I hope). We tried various combinations of devices, arrangements of pillows and cushions, tight wraps, to keep her lying on her back, but babies are remarkably, if involuntarily, wilful (or she was, anyway, and remains to this day).

I worry about very few things, but for the first few nights we'd regularly get up to check on her, and literally be holding our breath waiting for her to expel hers.

Out of necessity the every-parents-SIDS-fear, from allowing the baby to sleep on their stomach, had to be removed from our psyche so that we could continue to function day-to-day.

Said baby is now, thankfully, a semi-healthily functional teenager. As functional as teenagers get anyway :)