Didn't happen to me but I do have 1 recent relevant anecdote to which I was witness.
It happened in me3's multi, I had stumbled on a good group and we were acing the normal difficulty, so the lobby leader and his friend (both on mic) switched it up to hard figuring we could handle it.
First few waves went well, we had some nice teamwork and class synergy so blitzed through first half.
We could handle the odd duo of banshees mixed in with brutes and fodder, but when we hit lvl5 the game spawned a trio of them from the start and more came soon after.
It certainly didn't help that the map we randomed was that dark enclosed refinery with plenty of corners and close quarters fighting.
As lvl6 started, we were immediately assault by 4 banshees along with the standard complement of brutes and marauders. They completed routed us and we had to fall back, but were unsuccesful and got cut down 1 after the other till only the 4th was left, the salaran engineer.
He was one of the 2 friends on mic and here was his last words as he was hounded by the reaper forces:
"Multiple banshees! Problematic!"
*fires off 2 rocket launcher shots to kill 3 banshees*
*2 more appear*
*fires a shot to take them both out*
*3 more appear*
"Out of rockets to kill banshees with! Even more problematic!"
Guy said it in mordin voice too
Shame you can't chat to others without mic in me3, those were fun peeps that even complimented my krogan (said he looked omnimous in the red light <3 )