When Hoverfly is running in Diff mode, the "AddDiff()" function writes to the shared "responsesDiff" map without any synchronization (no mutex). When multiple proxy requests are processed concurrently (the normal case for any proxy), the concurrent map writes trigger Go's built-in race detector which causes a "fatal error: concurrent map read and map write", immediately killing the entire Hoverfly process. This is trivially exploitable by sending multiple simultaneous requests.