The matter-antimatter integrator control aboard the USS Enterprise.
The matter-antimatter integrator control was a component aboard Constitution-class starships which allowed crewmembers to monitor and modify the power-generation activities in the warp core. Located just off the main engineering section, it also contained the emergency overload bypass, which activated almost immediately in the event of an overload.
In 2268, Losira managed to sabotage the Enterprise's engines by fusing the bypass control, causing the ship to accelerate to wild and uncontrolled speeds. The defect was only corrected with Chief Engineer Scott managed to manually disrupt the antimatter flow. (TOS: "That Which Survives")