You don't see a lot of HTTP Content-MD5 response headers, but I just discovered some piece of code that generates unpadded base 64 versions.. i.e. a 22 byte:
I figured HTTP followed MIME rules and required a multiple of 4 characters.. i.e:
Weirdly, after checking the relevant specs it isn't actually clear to me if the = pad is required. I'm probably missing something obvious. But as this topic generates absolutely 0 google juice, this post is a public service announcement - expect both versions in your clients.