My own mum pulled this one on me. We were talking on the phone and she could hear my daughter playing in the background. As one point it dawned on her that she could not hear my wife.

  "Is Susan out?"
  "Yeah she's gone out to [I forget what]."
  "So you're home babysitting?"
  "No, it's just Alice here, no one else."
  "???"

In an extremely polite but pointed way, I informed her that I was her father, not her babysitter. She never made that mistake again.