Early last year, New York genealogist Mary Todisco responded to a post on a surname message board. To her delight, the gentleman who posted was a cousin. The two exchanged research — Todisco's was thorough; her cousin's was bare-bones.

Two months later, a woman looking for the same family posted on the board. Todisco replied, noting their common ancestor lived in the 1700s. "What I got in return was a curt ‘I don't feel our families are linked,’ and then she proceeded to tell me she'd been in contact with someone who'd done extensive research. Guess who it turned out to be?" The cousin had forwarded Todisco's research, claiming it was his own.