It was the sort of bad news every patient fears. Merlin Erickson, a 69-year-old retired engineer in Abingdon, Md., was told last year that a biopsy of his prostate was positive for cancer.

Mr. Erickson, worried, began investigating the options: whether to have his prostate removed, or perhaps to have radiation treatment. But a few days later, the doctor called again.

As it turned out, Mr. Erickson did not have cancer. The lab had mixed up his biopsy with someone else’s.

“Obviously, I felt great for me but sad for that other gentleman,” Mr. Erickson said.

The other gentleman was Timothy Karman, 65, a retired teacher in Grandy, N.C. At first, of course, he had been told he was cancer-free. The phone rang again a few days later with news of the mix-up and a diagnosis of cancer.