1 week ago user has asked me to display a custom link on Account that would point to another system we use. Really straightforward stuff, idea being that user is already logged in to both SF and that system.
URL should look like
Custom link, content source "URL", "display in new window", UTF-8 encoding.
URLENCODE promises to replace spaces with %20 but
The other system doesn't like plus signs, these really have to be percent-encoded spaces. A percent-encoded plus sign in the #2 is... wow, just wow. 3rd one is even better.
EDIT: Got consistent results in Firefox, Chrome and IE although I don't think it's a case of browser trying to fix it for me.