Sarah said that she later joked about the episode with "friends" J K Rowling and television presenter Emma Freud in the 10 Downing Street kitchen.
It was one of the dozens of "sometimes surreal" moments Sarah had during her almost three years at No 10 as the WPM (wife of the Prime Minister) she disclosed in a newspaper interview, the Telegraph reported.
No further details were provided about the incident involving the 47-year-old Italian PM, who is currently embroiled in a sex scandal and is to face trial next year on allegations of abuse of office and paying for under age sex.
In another revelation, Sarah said she was forced to take a different flight from her husband to Washington DC, where she had to change for a Presidential Dinner in the aircraft lavatory.
Despite hosting world leaders, celebrities and royalty, the only time she was ever "star-struck" was when she met Barbra Streisand, the American singer, said the mother of two whose memories are being serialised in a national newspaper next week.
According to her, she tried on dozens of occasions to not fall down in front of world leaders while wearing evening gowns.
She also disclosed that when US First Lady Michelle Obama visited London, she told her: "At last, a normal size person".
In an interview published yesterday to publicise her book, Sarah admitted that her main regret was not doing enough to help support her husband's tenure and not speaking out during his most important political fights.
"When I look back on it, maybe the one thing I would have changed about my role is that I would have spoken up more," the 47 year-old told Daily Mail from her charity office in central London.
"When things were going wrong, and I was reading stories about him that simply were not true, I wonder whether I should have got more involved — to say: 'this is just not the case'.
"The one thing that I could have done to make a difference was to be more vocal."
In her diaries she also talks movingly about the death of their daughter Jennifer, who passed away at 10 days old in 2002.
The couple "escape" to her grave when they need solace, she said. She wrote about the experience of "entering a long, never-ending tunnel" of grief.
In other moments, she admitted to "little episodes of exhaustion," where in one incident was given oxygen from a cylinder that was part of Prince William's security detail.