1. Dehydration. It doesn't matter if you're playing water polo or biking the Tour de France; you need to be fully hydrated on and off the field for your body to work properly. If you don't, you risk cramps, losing focus, declining mental prowess, and impaired muscle movement. You become slow, uncoordinated, and unable to make important decisions. You open up your body for injury. On the field, you should be drinking 6-8 ounces of water every 15-20 minutes. Off the field, drink at least eight ounces of water eight times a day. More is better off the field but try not to overhydrate on the field as that too will slow you down.
2. Faulty or ill-fitting gear. A helmet too big will allow the head to jostle around on contact instead of comfortably padding it. Pads that are too small won't cover as much of the body as need to be covered. Make sure that your gear fits well and is complete.
3. Unsafe playing environment. The environment you play on plays a major part in your safety. A field full of potholes is waiting to cause an injury. A concrete surface won't be as forgiving as grass. Inspect the environment you play in. Make sure it matches standards of quality before starting a game.
4. Competing at too high a level. A 140 pound featherweight cannot handle a match against a 300 pound heavyweight. The other guy has over double the bodyweight to throw around, making the featherweight an easy target. Competing against others at mismatched skill/maturity levels will get you or the other guy hurt. Play hard with people more equally matched with you to avoid injury.
Take care to follow these suggestions when playing sports. Not only will you perform better, but you'll stay safe while doing it.