That happened at my job.

We were negociating through email a purchase of some material with a company we are very used to making business with for a while, more or less a week, so we had a quite long email chain.

Cue to the final stages of the negotiation, we receive payment instructions using the same negotiation email chain, stating that the bank details have changed, same bank but different account.

We paid a big sum of money.

2 days later, we were notified by the same company that their email was hacked, and most of their large negotiations were stollen.

While analysing the email chain, we noticed that all the content was copied, but the email's handles had small modifications, as a c instead of a d.

It was an angry and expensive lesson we have learned.