I only ever had to use the Aldis Lamp once during 17 years of Aerodrome Control. Fortunately, someone was standing between me and the window I was aiming through.
"It's not working...You're not going out!" he said.
That was how we found out that the cleaning-lady unplugged both signal-lamps each morning, (after we had checked them at 0600L), to plug little 'Henry' in.
And no-one had noticed for about 2 years!