Pattern overload is the overuse of certain muscles by performing the same motion repetitively, usually in a manner that isolates some body parts while neglecting others. Its common among athletes who perform the same movements often (like a pitcher throwing a baseball) but also workers performing some sort of manual labor. Soft tissue like muscles and tendons that are overused begin to breakdown, causing inflammation and pain. To compensate your body starts recruiting other soft tissue to help out, sometimes leading to poor mechanics and cascading injuries.
Pattern overload can also occur if you regularly use weight machines at the gym. Most machines isolate one motion in just one plane of movement, working just one muscle while neglecting the stabilizers and other muscles that help with that particular motion. Not only is this sort of training impractical for real life, it can also lead to overuse injury, especially if you do the same routine on the same machines several times a week.
The best way to prevent pattern overload is to avoid machines and repeating the same workouts often. This is one reason we always preach functional movements using free-weights, body weight and resistance bands, as they distribute the work across many muscles. Its also why we work hard to design for our clients balanced workout sessions that address different movements. The key is to keep shocking your body.
If you are an athlete who has no choice but to repeat a motion when you play your sport then its important to factor in enough rest to let those muscles recover. A common cause of injury we encounter among competitive athletes is them using a certain muscle several times a week while they play their sport and then working on the same muscles at the gym during their off days.