In Halo: CE, on the first level that the Flood is revealed.
Now that I think about it, that entire level was basically an "Oh, Shit!" moment.
From the minute the level starts, you get a feeling that something is very, very wrong. Then, while you're watching the last moments of Private Jenkins, the "Oh, Shit" alarm goes off non-stop.
The dialogue between my cousin and I during the rest of that level consisted of:
"Oh, shit. Oh, shit. Oh, shit."
"Die! Die! Die!"
"Oh, hell. Oh, crap. We're gonna die. No way we'll beat this!"
And then when a really big mob of Flood came after us:
"Shiiiiiiiiiiiiiiit!" *said whilst spraying two clips of assault rifle bullets*