Just as with a real launch, technical glitches occur. In an earlier test, known as a wet dress rehearsal, in which the entire countdown was simulated except for igniting the engines, almost everything went well. But in the last few seconds, one of the propellant valves did not close as soon as expected. It turned out that the temperature was a bit lower than predicted, and that made the valve a bit harder to turn. The software has since been adjusted.
A worst case would be if a malfunction led to destruction of the booster. That would add years of delay to the program and renew calls for NASA to consider alternatives.