Shifting the point of view revealed a rest constraint that is probably so much taken for granted that it is not talked about very often:
"The only thing that is required to be static for a resource is the semantics of the mapping, since the semantics is what distinguishes one resource from another."
(Section 220.127.116.11 Resources and Resource Identifiers of Roy's dissertation)
This is a guarantee the server has to make that is actually possible to adhere to because no evolution scenario of a server can possibly force it to change that mapping. The other two things that are not subject to change are (obviously) the uniform interface and the use of descriptive message semantics.
Besides these three aspects there is nothing in a networked system that servers can guarantee to clients and REST emphasizes that in an incredibly honest way. The client may depend on the above three aspects to remain true for the entire lifetime of a service but must plan for anything else to change.
What is the take-away? Two things mostly: you are not doing REST unless your system does not adhere to the above and you should exploit the stability of the resource semantics as much as possible in your service design.