In Washington, D.C., Kathleen Smith, a chief marketing officer in her late 50s, often texts with her husband and a colleague she calls her “work husband”—and frequently mixes them up.

“I’ll send a message and I’ll get a response that says, ‘I think you meant to send it to your other husband,’ ” she says.

In particular, Ms. Smith says her colleague doesn’t appreciate being called “pumpkinbear,” a term of endearment she uses for her actual spouse.

At a recent job in Columbus, Ohio, Derek Drake, 33, was used to texting his wife daily to confirm when he arrived at work, saying “I’m here. I luv u.” On one occasion, he sent the message to his boss instead, who he recalls dryly texted back: “I know. I’m sitting right beside you.”