A random war-story from B2B integration land... One customer system we had to interoperate with required an are credentials to be provided via an HTTP header. However it kept giving errors, until I realized that it only recognized the HTTP header when "authorization" was entirely lower-case, a sensitivity in violation of the HTTP spec.

Since our stack's own HTTP client libraries always used title-casing on the wire, we had to find a way to slot in a special exception, code to modify that header before it went out.

Another fun one is all the services which say their mime type is "application/json" while emitting generic (non-JSON) error message pages. So our logs are full of JSON decoding errors, rather than something describing what actually went wrong on the other end.