The 727 engine anti ice panel had a rotary switch to check the valve position agreed with switch selection
With the EAI switches forward all engine anti ice valves opened and correct operation was confirmed with three green lights, turning the switches off would see the lights go out then turn green again once the valves closed
The green lights showed the valves agreed with the switch position
On one occasion the FO tested this panel but then turned off the rotary selection switch while leaving the EAI switches in the forward, on position
All lights were out so everything appeared normal, it was only during the ensuing climb that the lack of performance was apparent, the EAI switch position was noticed and turned off, everything returned to normal
That switch panel was a poor design, should not have been possible to have all lights out with EAI on