In this case I was fixing a new roof that was leaking. See - How to find a roof leak in a new roof or old roof.
On this particular roof I had discovered that rain was running down the roofing nails.
So I measured the approximate distance to where the leak was occurring and started lifting up all the shingles near the ceiling leak.
And I found some nails that had been nailed down too much destroying the shingle which allowed the water to drip down.