Royale:

Halo:CE, in the level The Maw.  I remebered i completely missed the area, and went on to beat the game.  It was only until i replayed the level several months later did i come across the area.  It was right before you had to destroy the exhaust ports, and Cortana says that there's an Munnitions room where i can refill w/ explosive ammo if i run out trying to destroy the exhaust ports.

 Well, while searching around, i stumbled across the room, and was like, 'Oh cool!!  Rocket Launchers, Grenades, Oh my!'  I went to pick up rocket launcher ammo, and heard a sliding sound from an end of the room.  I went to check it out, but i saw nothing.  I looked around a little, and i saw a part of my screen distort.  Then i recognized a small blue light where the blur was at.

It was a cloaked Flood combat form.  I just completely freaked out and went nuts.  The door at the end of the room slid open again and more cloakies came in.  I remembered i just blast away w/ whatever i had at hand, just running like a maniac around the room.  I swear my heart skipped several, several beats.

 Yeh those invisible infected flood freaked the hell outta me too.