Hmm. Probably in Battlefield Bad Company 2 when me and my friend were alone in a squad in a game of hardcore squad deathmatch. He was sniping as a sort of "overwatch" and I was an assault, going and sneaking up with a shotgun on the 3 other full squads who had been very organized. We had been playing with these people for a while and were holding our own when my friend mentioned he had seen some movement in the window of a house that I was near.
As I ran up to the door, I tossed a grenade through the window. One guy was downstairs, who I tried to shoot and missed. Another guy came running down the stairs to get away from the grenade and I shot him before he even saw me, then finished off the other guy.
Just as I was about to reload, one of the two guys upstairs tossed a grenade down, right at my feet so I had no other choice but to rush up into who knows what. When I got up there, I could only see one guy and, once again, I missed with my shotgun. Now out of bullets, I whipped out my pistol (the WWII one, can't remember it's name) and emptied the entire clip in a matter of seconds in his general direction. Somehow they didn't kill him so for the next 10 seconds we ran around each other, trying to get a knife kill and both failing miserably until I finally got behind him and finished him off.
At this point I was so relieved that it never occurred to me to check the other room for the 4th guy, so understandably, I was surprised when he walked out on me, machine gun blazing. Thinking that I was about to die, I screamed at my gun to reload faster as just enough bullets hit me to bring me to the brink of death. Just then, there was a far-off gunshot heard over his wild machine gun and he fell to the floor, sniped in the head my my team mate.
*sniff sniff, best moment ever
tl;dr after killing 3/4 of a squad in BFBC2, my team mate saved my life as I reloaded with a sniper headshot from halfway across the map through a window