How big do you think the envelopes were? The BBC reports:

An Aberdeen woman who asked for her bank statement was sent those of 75,000 other customers.

[…]

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.

What control would be the best fit for this mistake? Match the account address to the mailing address? Require customer re-verification if the number exceeds a certain buffer of statements?

Perhaps what is most strange about this case is that it happened through the regular post. We all worry about exposing accounts in the digital world because the controls are virtual, so it seems hard to believe that a system could screw this up in the paper/physical world.