A few years back I had mail to an address forwarding to a team at a different company. It was used to cc them on tickets, basically so they could make changes on their side when certain requests came in. At some point they decided to forward these directly into their own ticket system. Each email from our ticket system would forward and land in their own ticket system that would strip off the identifiers from our ticket system and create a new ticket which would reply to our ticket system, creating a new ticket on our side and so on. Unfortunately that queue on our side wasn’t in use by humans so no one saw the exponential mail storm building. Many mail servers have relay loop detection based on IDs in the headers but reply loops are a different beast. Once we realized what was going on it was simple enough to black hole the various addresses involved but it was pretty cool to watch our ticket and mail systems DOSing each other in real time.