“This meant some customers received emails informing them their new premium was now $2700 instead of $27.00.”
A homegrown SMTP client wrapped long lines and left a period at the start of a new line. The SMTP spec says to double it. The code didn’t, so the server ate the dot. One team fixed it. Another team copied the old code and told customers their $27.00 premium was now $2700. Rolling your own protocol client means rediscovering every bug the RFC already wrote down.