The Titanic actress is currently working alongside pop superstar Timberlake in the new, untitled Woody Allen movie, and she became so caught up with the production, she forgot who she was talking to.
"There was a fun little moment (on set)," Kate recalled in an interview on U.S. breakfast show Today.
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"I hum a lot on set and I do kind of just absent-mindedly sing between takes and then he was doing the same thing, and I turned to him and said, 'Oh, so that will be a thing (we share). So do you sing?'"
The British star immediately regretted her comment, and she spent the next few moments berating herself for the silly remark.
"Then I was like, 'Of course he sings, Kate! Of course he sings! Why did you just say that?'," she laughed, before gasping, "Can you imagine?"
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The project, which is currently shooting in New York, is the latest to make Kate's teenage daughter, Mia, jealous of her mum's work as the actress also recently starred opposite Australian hunk Liam Hemsworth in The Dressmaker.
To make it up to the 15-year-old, Kate's child with first husband Jim Threapleton, the star has promised to invite her to the set one day.
"It does seem a little mean, doesn't it?" Winslet mused. "It's OK, she loves me. She forgives me, and she has said, 'Mummy, if you don't let me come onto that set I'm never going to speak to you again!'"
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