On the circus act where the clown rode the top of a 60 ft pole that drifted after the slow mode was told to stop, the locking valves were causing the problem. The circuit symbol shown in the schematic showed a spring returning the valve to the closed-and-locked position and a solenoid would force it open. The valves were rated for a 30 gpm flow at a 140 psi pressure drop.
The valve manufacturer's catalog described the valve as “internally piloted, bi-directional, load-holding valve with 7 drops per minute maximum leakage at 3500 psi.” What the catalog did not say was that it required a 30 to 40 psi pressure drop across the valve to provide sufficient pilot pressure to ensure the valve would close completely. The valve also had a preferred flow path.

The manufacturer was consulted and they provided modified valves with stronger springs that did not require any pilot pressure to help shift and hold the valve closed.