FNB has confirmed that the accounts of some of its customers showed incorrect balances.
On Tuesday afternoon some of the bank's customers started to post on social media that their bank balances had appeared to increase.
"Anyone receiving free money from FNB? My balance confused me this morning," posted Twitter user @wamelona.
In an update on Wednesday, FNB said that the incorrect balances were due to "delayed debit card transactions for purchases by impacted customers".
On social media, FNB customers posted that the incorrect transfers appeared to have been reversed.
"Due to client confidentiality, we will be communicating directly to impacted customers. FNB apologises to impacted customers for the inconvenience," said Daniel Kaan, from the bank's transactional banking division.
Kaan said the issue had been "resolved" but did not provide further information.
The bank said customers with specific queries should contact the bank.