For me it was Rescue on Fractulus (AKA Rescue Mission, AKA Behind Jagged Lines). For those individuals who have never heard of this brilliant game, I played it on the Atari 8bit computer, and you shoot out of a space station and clear the skies above the planet, killing aliens and then you land and fetch crashed pilots. This was an early first person shooter with fractally generated landscapes (hence the name of Fractulus). The first few levels were straight forward, you land close enough to the crashed ship, turn off your engines and the pilot will run out to you and knock on the door. You open the hatch, let him in and then back to the base.

As it gets harder you may notice the pilot has a green helmet. These are enemy aliens, you turn the engines on when they get close and it vaporizes them!

But further along, they get tricky withthe white helmets... and if you let them get close enough they pop up on your screen and alarms go off scaring the crap out of you.

The first time this happened to me was when I was playing with my brother and we literally fell backward out of our chairs and of course died in the game.