Kiwibank was forced to text its customers asking them not to upgrade to the newest iPhone software - because its app couldn't handle it.
And it wasn't the only Kiwi company that stumbled at the hurdle when the new software was released last week; accounting software firm Xero had the same trouble.
Kiwibank spokesman Bruce Thompson said the problem affected businesses worldwide.
"A late change in Apple's rollout of its new operating system iOS6 on Wednesday night changed the way Safari caches web services - and it had a global effect," Thompson said.
"It meant that customers who had upgraded their Apple device to iOS6 had customer experience issues with cached balances and some authentication problems."
He said there was no financial risk to customers.
"Once we noticed the issue, we got in touch with all affected customers and gave them a workaround, and put in a full fix on Friday. We also got in contact with customers using Facebook and Twitter over those couple of days. There are no further issues with our app on iOS6."
Xero spokesman Matt Vickers wrote on the company's blog: "Unfortunately, iOS 6 appears to have caused some problems with the most recent version of Xero Touch, which is affecting access via the iOS app and via the browser.
"We'll be submitting an upgrade to fix the problem as soon as we can and it will be available as soon as Apple approves it." An update was done that did not require a release to the App Store, but customers were told to reinstall the Xero Touch app if they were still having trouble.
Apple referred queries to the companies themselves.
Freelance designer Iain Spanhake, who was not responsible for the Kiwibank or Xero apps, said it was likely an error by the software giant.
"By the looks of it, it is Apple's fault as the bug is 'embedded' within the new version of their Safari web browser, so it is an error that Apple needs to sort quickly ... An update should be posted in the App Store shortly so when people update, the problem should be fixed."
He said most apps tried to send and receive multiple data "packets" at once, but the bug allowed only one at a time, causing delays.
"A lot of people have been complaining of seeing a little spinning icon on their screens - that's the web app trying to talk to the server, but only sending and receiving one data packet at a time."