A branch bank had just been robbed in College Point, Queens. Inspector Michael S. McGrath of the New York Police Department ushered me into a small grocer’s next door. A few boxes were arrayed haphazardly on nearly empty shelves. This place was almost surely a front.

The inspector approached the counter. I didn’t notice him casually placing his right hand on it.

Turning to me unexpectedly, he asked, “What have I just done wrong?”

I had no idea.

Inspector McGrath looked down his white shirt sleeve to the tips of his fingers, whose prints were now on the counter top. “I may have just contaminated the scene,” he said.