Carrie Lam mocked John Tsang for having a "clean desk" - one that is free of documents.
Tsang replied that it was smart work.
When asked to name something she admires about her rival, Lam said she often envied his clean desk while serving under him as secretary for development. And she said it was never possible for her to have such a desk, as she had a lot of work to do. But the "clean desk" jibe may have quickly backfired. Bruce Lui Ping-kuen, a voter from the higher education sector, responded shortly afterwards by posting online a photo of President Xi Jinping and his desk - which also had few papers on it.
"I wonder how Carrie Lam would rate President Xi's desk?" Lui's caption read.
Netizens joined in Lui's thread and posted photos of clean desks of other famous people, including Barack Obama and Li Ka-shing.
Third candidate Woo Kwok-hing, who was also at the forum, shared a photo on his Facebook account afterwards from last year, saying: "My desk is also quite clean."