A roller coaster's sudden twists and turns can result in a dangerous blood clot or a torn blood vessel. This injury can trigger a stroke in both adults and children. People with high blood pressure or heart conditions are at greater higher risk.

A roller coaster becomes "unsafe" primarily through a failure in the triple-redundancy safety system: design, maintenance, and operation. Structurally, "metal fatigue" or stress cracks in the track or supports (often caused by rust in older steel coasters) are the most significant risks, which is why daily ultrasonic testing is a modern standard. Mechanically, a coaster is unsafe if its "fail-safe" braking system—which is designed to stop the train automatically if power is lost—is improperly calibrated. Additionally, "customer error" or "ride operator negligence" (such as failing to verify that a restraint is properly locked) are the leading causes of modern incidents. In 2026, engineers also focus on "G-force safety"; a ride can be inherently unsafe if its transitions are so abrupt that they cause whiplash or "grey-outs." Ultimately, a coaster is only unsafe when its rigorous daily inspection protocols are bypassed or when riders ignore safety height and health restrictions, as the machines themselves are designed to be among the safest forms of transportation in the world.