I'm going to regret this.
How often did that happen? And no generalities. Can you come up with a specific number in comparison to say a run? Because then you can mathematically point out whether not it's happening just as often now. It's a legit question, so try to answer it legitimately.
The ones that I can think of off the top of my head are....
Superman #1 Perry White sends Clark Kent to cover a war in South America.
Worlds Finest #255 - Clark Kent honks Morgan Edge off, and get punished by having to cover a corn festival somewhere in the Midwest. While he's there, he discovers there was a Batman-like legend in the area.
Action Comics #592 Clark Kent is doing a story a free clinic in Metropolis, here he discovers it's full of elderly people with impossibly extended life spans. He discovers the patients have traces of radiation in their bodies, and goes off as Superman to go find the source. This was the first part of the Sleeze story, and we all know how that ended.