BBCSome customers using Bank of Scotland, Lloyds and Halifax apps have been able to see other users' transactions on their accounts.
Lloyds Banking Group customers reported being able to view charges and payments from other sources on Thursday morning.
A Lloyds Banking Group spokesperson apologised for the issue and said the incident had been quickly resolved.
An investigation is under way.
One woman told BBC Scotland News she was able to see the accounts of six different users on the Bank of Scotland app, including some National Insurance numbers, over a 20-minute period.
Those included transactions from a pub in Newcastle, 154 miles from her home in Kirkcaldy, Fife, fees for using one card abroad and wage payments from a company based in England.
The 55-year-old also reported being able to view benefits payments from the Department of Work and Pensions (DWP), which use the National Insurance numbers of recipients as a payment reference.
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