Lloyds Bank customer Carl Lewis told the BBC opening up his app on Thursday morning to see transactions unconnected to him made him "very concerned about the safety of my details".
"It was the full history of the account that I could stroll through month by month, including direct debits to the DVLA where the car registration number is shown," he said.
Sam Harris, who has an account with Lloyds Bank, discovered the glitch at around 07:20 GMT.
"I saw nearly £3,000 had gone in and thought that must be someone's salary and it said where their salary had come from," he said.
He told the BBC it had made him "really worried" about his information or details being similarly disclosed to other users.
Meanwhile another user, Asha, described feeling panicked after seeing unknown transactions on her app - especially as their figures appeared to match the totals of her back account.
"I assumed I was hacked or a fraud had went on," she told the BBC.
"I genuinely thought someone had cloned my details - one transaction was by someone who bought a car. I thought they'd spent £8000 of my money."
She added the experience had left her feeling "almost traumatised".
"This morning we incorrectly showed transaction information from some accounts to other customers in Internet Banking and the mobile app," the Lloyds Bank account on X has told some concerned users.
"We're sorry this happened. This issue was quickly identified and resolved."
Outage tracking website Downdetector saw a spike in reports of issues with the Halifax and Lloyds apps between 07:00 and 09:00.
A smaller spike was reported on the Bank of Scotland app, according to the website.
Lloyds Banking Group says it is the UK's largest retail and commercial banking provider, with 26 million customers.
This is not the first time the Lloyds Banking group's apps have experienced issues.
The Lloyds Bank, Halifax and Bank of Scotland apps were affected by technical issues during wider UK banking app outages on payday in January and February 2025.
Those glitches saw thousands of reports from users to platform outage monitor Downdetector, and calls from consumer groups and officials for banks to boost their resilience.
About 700,000 of those affected by payday outages were Lloyds Banking Group customers, its chief operating officer told the chair of the Commons Treasury Committee in a letter.
Those banking app issues left customers unable to view their own recent transactions or make payments - as opposed to exposing the financial details of other people.
Markos Zachariadis, professor of financial technology and information systems at the University of Manchester, told the BBC it was an "unusual" incident, and suggested it might be related to the growing sophistication and popularity of digital banking.
"As data openness becomes greater and more complex architectures arise such issues can become even greater," Prof Zachariadis said.
The Lloyds Bank account on X has sought to reassure customers expressing concern about the security of their accounts or information.
"We can assure you that nobody had access to your accounts," it wrote in posts seen by BBC News.
"We're currently reviewing what happened to ensure this cannot occur again. Protecting our customers' personal information and account security remains our priority."
An spokesperson for the data regulator, the Information Commissioner's Office (ICO) said: "We are aware of an incident affecting some online banking services and we will be making enquiries."
A spokesman for the City regulator, the Financial Conduct Authority, said: "We're in contact with Lloyds Banking Group to understand what's happened and how it's being resolved.
"We expect firms to protect customer data and be able to respond to and quickly recover from disruptions."
Additional reporting by Mary Litchfield, Rebecca Curran, Andrée Massiah and Valmira Gjoni