Which celebrity endorsement led to a disastrous product launch when they publicly used a competitor's product instead of the one they were paid to promote?

Celebrity endorsements can make or break product launches, creating iconic partnerships that last decades or embarrassing misalignments that companies would rather forget. Test your knowledge of famous people and their product partnerships with this challenging question about a notable product launch mishap involving a celebrity.

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Alicia Keys as BlackBerry's Creative Director while tweeting from an iPhone
Taylor Swift promoting Diet Coke while being photographed with a Pepsi
LeBron James advertising Samsung while posting from an Apple device
Oprah Winfrey promoting Microsoft Surface while tweeting from an iPad