An Aberdeen woman who asked for her bank statement was sent those of 75,000 other customers.
Stephanie McLaughlan, 22, was shocked when Halifax Bank of Scotland (HBOS) sent her the unexpected financial details by mistake.
Ms McLaughlan received several large packages in the post and said she was concerned it could happen.
HBOS apologised and said it was carrying out an investigation into the "serious" but "isolated" incident.
'Concern caused'
HBOS said in a statement: "We are treating this matter very seriously and are investigating in full.
"This is a very specific, isolated incident and we will take steps to ensure there is no security issue for customers as a result of this matter.
"We apologise for any concern this has caused customers."
HBOS was formed in 2001 with the merger of the Halifax and Bank of Scotland and has millions of customers.