This one was
so close to a moment of ownage...
The scenario: a Very Large UFO was detected over Europe and destroyed the plasma-armed Interceptor sent to bring it down. It eventually landed in Northern France, and the only team that could make it in time was Bravo Squad, made up mostly of rookies after a disastrous terror mission. Only half the team was armored, and other than two heavy plasma guns and a single stun launcher everyone was armed solely with laser rifles.
Thankfully, it was the middle of the day when the Skyranger landed. The UFO was massive, and Snakemen were wandering outside of it. Amazingly, Bravo Squad dealt with all visible hostiles without a single casualty and proceeded to clear every building on the map while some of the team took up spots near the UFO's entrance in preparation of breaching.
Suddenly, one of the doors on the craft opened and an insectile black shape came out. Crysalid! The rookies opened fire while it charged directly towards the ranking officer on the squad; a sergeant who had, on her first and only other mission, made five kills with accurate long-ranged fire. Several shots hit the beast but it remained standing, and in what were undoubtedly her final moments the sergeant managed to get off a shot right before it could strike. Astonishingly, the crysalid actually turned tail at that point and tried to retreat, though further reaction fire killed it. This all happened in a single enemy phase, by the way.
Well, knowing that these monsters were on board the ship, there was no way I was sending Bravo Team in. The squad formed up within sight range of the entrance, hunkered down behind what little cover there was, and prepared themselves to blast anything that moved. And it worked! Alien after alien, Snakeman and crysalid alike, were gunned down without mercy. Some of the snakemen managed to get shots off, one of which hit an unarmored rookie who remained standing. At this point, I was suitably impressed by Bravo Squad.
Then finally, the door opened one more time. A couple of shots hit the shadowy figure, but it stayed alive. It fired the most feared weapon in the alien arsenal, a super-charged rocket that could turn corners and navigate around obstacles. 4 members of Bravo Squad died in an instant, and the alien was shot and killed with a vengeance. The mission ended immediately afterward.
tl;dr: Fully intact very large UFO, loaded with crysalids, perfect mission ruined at the last possible moment.