Clear thinking is more difficult in pressure situations. This is why coaches and athletes must constantly practice what they are going to do and how they are going to respond in pressured-packed situations. So that more blood is available to the large muscles of the body in preparation for strenuous physical activity such as fighting or running away, the digestive system shuts down. During this shut down, the acid in your stomach makes you feel nauseated which sometimes results in throwing up. This is a normal reaction to stress.
Caffeine tends to exaggerate the physical and mental effects of stress.
Knowing this, coaches and athletes should avoid caffeine products before entering potentially stressful situations. Under stress, the body produces adrenalin which provides a powerful, quick burst of energy sometimes resulting in superhuman feats. There are a number of stresses which are good. For example, being elevated to the starting team brings additional stress which most athletes would enjoy. Another example of positive stress is physical and mental training.