When German searchlight crews managed to cone a bomber, no more than four searchlights were to be used. However, other searchlight crews would get excited and sometimes more than a dozen lights would play on an RAF bomber. This had the effect of ruining the night vision and thus blinding the pilot and crew of the German night fighter trying to shoot the bomber down. German night fighters constantly fired flares as a signal to searchlight crews that friendly aircraft were in the sector and turn off some of the damn lights. This rarely worked.
(Source: German Night Fighter Aces of World War Two by Jerry Scutts)