A reader reminded me of this incident, and after some digging around, I've come up with the details.
It happened in the mid-80s. The chief of staff for Pierre Trudeau was Jim Coutts. During a media scrum, Liberal MP Bob Bockstael delivered a sealed envelope addressed to Coutts.

The problem was that he in fact handed the envelope to CTV reporter Mike Duffy in a case of mistaken identity.

What was in the envelope? The rumour was that it was the secret plan to win Liberal Party support in the West (at the time, the only MPs from the West were Bockstael and Lloyd Axworthy).
We'll never know, of course. Realizing that he was given a sensitive document as a result of an innocent oversight and not as a tip related to a news story, Mike Duffy delivered the envelope to Coutts unopened.
That's class.
An interesting contrast to the sort of thing we see today.