It’s a given that as a general rule, things never go exactly as planned. There are many factors that cannot be effectively predicted, and there is something called Hofstadter’s law. This law refers to the difficulty it takes to predict how long it will take to complete a task of certain complexity. The human mind is prone to visualizing the self performing feats with much greater ease than how it is in reality.
Criminals are not too bright, but they tend to pick easy targets simply because it poses less risk. As a result, things will be more likely to go to plan. When they get too confident and pick on anyone that seems remotely vulnerable, they run into the large risk of things going opposite of what was planned.