If a decent tree trunk is available, setting an anchor is a breeze. Life isn't always fair though. Often trees are not close enough and we need to make due with boulders or large rocks. Chocking may be the only way to set an anchor. All anchors should be tested under a bit of load before they can be trusted. With marginal anchors, you may need to set more than one to spread the load. There are several ways that anchoring systems can fail. That chock or large rock may shift under heavy loads. Improperly tied knots can fail. Older rope may stretch and break. Friction weakens rope rather quickly, especially polypropylene which has a very low melting point. A snapped rope acts like a snapped wire and can lacerate a person caught in its path. Pulleys or carabineers can become dangerous projectiles. Plan ahead, use dampers and brakes. Wear your protective gear like helmets and PFDs. Consider adding a direction change pulley or carabineer to take you out of the line of fire. Always have someone not pulling act as a spotter for any signs of failure in the system.
Here is a short video from our annual "Fun With Ropes" Winter class where we intentionally force a Z Drag to fail. Listen to the final pull and the very loud snap as our hardware hits the tree. In this class, we only had a few persons pulling and engineered an intentional break point. Under real conditions, that projectile would have been far more serious. This demonstration clearly shows why safety precautions are very important.