I have another good example. Please see this thread I posted to the Go mailing list a few months ago:

https://groups.google.com/forum/#!searchin/golang-nuts/s3/go...

The short summary is that Amazon S3 does not behave like a normal web server and expects HTTP path's to be "url encoded". S3 is pretty obviously something you would want to interoperate with. Go's http client (which isn't very strict at all) causes friction here.