"The F-4 had a sort of potential nasty where if the rear seater pulled the handle just before the front seater the rear seater would be going up the rails just as the front seat canopy ejected.... the rear seater would quite possibly "collect" the canopy with probably fatal results. "
This was in fact a real nasty - XV397 on 1 June 1973. The navigator who had initiated ejection first was struck by the pilot's canopy. Thereafter, the RAF Phantom sop was for the pilot to announce the need for ejection by doing so.