It does happen - there was a Whooper that just never left Martin Mere for at least one year (though I think that was an injured bird, so it was sort-of 'self-pinioned'!). I remember because I was in the 'In Focus' shop buying new binoculars and just as I was trying them out on the injured bird the first Whooper arrivals of the winter flew in - a good omen, as the salesman was keen to point out!
Were these birds on the 'reserve' part or in the grounds of the captive collection? If the former your memory could be correct, if the latter I think it's case closed I'm afraid!