The pair in question were playing in an event called the 'entente cordiale' between British (then) junior, university, open, women's etc teams and the equivalent french teams. They were soon to go on and win the European and World Juniors.
One of them was looking at something resembling this freak (I can no longer remember the exact heart cards):
Scoring: IMP
LHO dealt and opened the bidding and the auction started
2♠ 3♥ 4♠ 7♥
x
At this point he was feeling overjoyed, and just waiting to redouble and watch LHO's ace of clubs get ruffed off.
What's the worst that could happen next?
That's right.
Partner pulled to 7NT.
This was also doubled on his left, and went for 1700.
7Hxx would have made (the double of 7H was insane)
That's the biggest swing on one bid we've* ever seen.
*'We' because I was playing for university team in the same event and got to hear about this at length, repeatedly