Bobbie Gordon assumed Asda had made a typo when they emailed her to say the 11p piece of fruit would actually cost her Â£930.11.
Her credit card company's fraud team prevented the payment from going through, and text her to say her bill had reached in excess of Â£1,000.
Ms Gordon, of Sherwood, Nottingham, said Asda had since rung her to apologise and that the mistake was due to a computer error.
She said: "I didn't think they would actually charge me until I got an automatic text from my card's fraud department querying the transaction.
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"I told my seven-year-old 'you must really enjoy this banana, you must cherish every mouthful'."
Ms Gordon added: "Asda rang and apologised over the phone. They just said it was a computer error and they were very sorry it didn't get picked up on their system."
An Asda spokeswoman said: "Whoops, looks like we've slipped up here. Whilst our bananas are excellent, even we agree that they are not worth that much and clearly there has been a glitch in our system.
"We would like to thank Ms Gordon for keeping her eyes peeled and flagging this error to us and we are investigating to ensure it doesn't happen again."