Now here is something to ponder. Ever now and then I get offered a 'Once In A Lifetime Opportunity' to win a holiday to somewhere exotic, or buy my dream home in the sun or something. However, since I seem to be offered these things reasonably often, they are not really once in a lifetime opportunities.
Things I expect to happen once in my lifetime are, on the whole, rather more mundane, such as buying a bag of 1000 paperclips, installing cavity wall insulation, or reading 'Catch-22' (please never again).
Just a thought.