I can't provide any exact recount, but I can describe a scenario I've had several times in a variety of missions (and various games).
You begin the mission. Everything goes well. In fact, everything goes incredibly well. Everything falls in place and you're mowing down the aliens left and right. Every shot lands, every grenade strikes true (or, well, as true as an area effect weapon strikes). Every bit of alien opportunity fire missing or rolling an attack score that's too low to punch through your armour. It's just a perfect run.
By the time you start hunting down for the last few (some of which are panicking), something goes wrong, the scales are suddenly tipped and you are slaughtered. It could be anything. Return fire actually hurting you, a grenade, mind control/panic, you running out of TUs just as you spot an alien. Anything.
Your soldiers start to get cut down one after the other and the rest start to panic. Most of the casualties are a direct result of them not being able defend themselves. The mission is suddenly in a complete state of chaos and there's nothing you can do to stop it until the last soldier is either slaugtered or found a a way on to the ship and you issue the orders to dust off.
The result? A perfectly executed mission is suddenly turned into a complete and utter failure. It's not a pretty sight, but hey, it happens to even the best of us.
NKF, narrow minded fuddy duddy who refuses to let go of the past and will not accept anything newer than 1979.