The ex-accountant, who won the seat from former Conservative Zac Goldsmith, was caught off guard when, in reference to Lib Dem leader Tim Farron's support for a second EU referendum on the terms of Brexit, Hartley-Brewer asked why there shouldn't also be a second by-election.
Hartley-Brewer pressed Olney on whether it is right for a party to change its policy after a referendum.
Olney responded by saying there "wasn't a clear manifesto set out for the terms of leaving", a claim disputed by Hartley-Brewer, who said it was very clear Brexit meant leaving the single market.
The MP's PR adviser interrupted, saying: "Erm, really sorry, but Sarah Olney has to leave now."
"No, she doesn't!" was Hartley-Brewer's stern response.
The serious point is that Olney and her party failed to communicate their reasons for supporting a controversial policy, and instead appeared muddled. It's a lesson that the country's newest MP, and her party, won't want to forget soon.