Let's see...my most recent example of how things can go wrong when flying happened when I was flying from Charleston to Huntsville in July. Normally not a big deal. Two flights with a connection at Atlanta. The flight from ATL to HSV is less than 30 minutes once the airplane rotates (takes off). I arrived at ATL at around 3pm with my flight to HSV scheduled to depart at around 4:30pm. It was stormy around the airport and flights were being delayed. This was a problem, but actually the big problem had started earlier that day in the northeast when bad weather caused major delays at key airports. The cascade had started. The airplane that was supposed to come to ATL and then fly me to HSV was late arriving from up north. I ended up boarding a rerouted airplane at 9pm and got to HSV about 6 hours later than I should have.
They also had to cancel a couple of flights to BHM and combine them for a later departure time.
This is why I always tell people to arrive a day early.