Natwest customers reported seeing their money "disappear into thin air" on Friday as the bank struggled with issues on its app.
The bank said it was aware of an issue which meant some customers were having problems with its mobile banking app and with online banking running slowly.
In some cases, customers were having issues with transfers being delayed, it said.
The glitch came on what is payday for many people, being the last Friday of the month, and a day when many are checking their finances as they make preparations for the Bank Holiday.
One Twitter user wrote: "When you transfer money from a Natwest account to another and it just disappears into thin air." Another described it as an "absolute disaster".
An RBS/Natwest spokesman said the issues started this morning but were fixed by the middle of the afternoon. He did not confirm what had caused the problems, but said the bank was working hard to resolve them.
He said: "We are aware that some customers are currently experiencing issues with our mobile app and online banking running slowly.
"Some customers are also seeing issues completing transfers on accounts. We are working hard to resolve this and apologise for the inconvenience caused.
"Our Natwest and Royal Bank apps and online banking are operating, but some customers are experiencing slowness trying to access them.
"Telephony and branch banking are both available. We are seeing that some customers are having issues with their transfers being delayed."
A spokesman added just after3.30pm: “Our mobile apps and online banking are now running as normal and delayed payments are starting to credit customer accounts. We apologise for the inconvenience caused.”