every random encounter in any rpg, especially when you're low on supplies and a miniboss level enemy appears...
like when you exit a cave in pokemon and gary is sitting outside
scenario one;you're in a building that has a bunch of small roomsyou press a button and it opens up progression to the next levelyou walk out of the room, then you hear radio chatter."priority one anti-citizen. engaging, spread out."and you walk with you shotgun out, turn a doorway and this pops up in your face out of nowhere;AND THEN YOU HAVE THEM IN GROUPS ON THAT PART WHERE ALYX IS DOING SOMETHING AND YOU"RE IN A PLAZA WAITING FOR IT ALL TO BE OVER WHILE FENDING HUNDREDS AND HUNDREDS OF THEM OFF.