"My son thought he was a millionaire for a few hours.
" … My husband messaged me with the photograph of his bank account details. It took me a wee while to click.
"He's going to me, 'I'm gonna order a Porsche' and I said, 'Don't be daft, that's not our money, we need to get this sorted'."
The bewildered mum said her son had enjoyed being a millionaire while it lasted.
"It was like that for a good few hours. For an 18-year-old being a millionaire for the day, he just can't believe it," Gillespie said.
"He just wanted to go out and spend, but if you do then you have to pay it back.
"He thought all his birthdays came at once this morning. It's mad."
However, it didn't take long for Nationwide to fix the error.
In a statement provided to the Daily Mail, a bank spokeswoman said Gillespie wouldn't have been able to purchase a Porsche as the funds had never actually made it into the teen's account.
"The customer paid the cheque in at the branch and unfortunately the wrong amount was keyed," the spokeswoman said.
"The error was noticed straight away and the balance was corrected in a matter of seconds.
"While the amount was shown on his balance, cheques do not clear straight away, so the funds were never in is account.
"Sadly he may have to wait a little longer to buy the Porsche he wants."
- News.com.au