What is rightmost sentential forms?
I found other answers here confusing, so let me explain it clearly here-
Definitions:

A sentential form is any string derivable from the start symbol. Note that this includes the forms with non-terminals at intermediate steps as well.
A right-sentential form is a sentential form that occurs in a step of rightmost derivation (RMD).
A sentence is a sentential form consisting only of terminals

The examples in your question are all sentences. And since any way of derivation (including RMD) ultimately leads to a sentence, technically all sentences are right-sentential as well as left-sentential forms. So there isn't any counter example which you asked for.
Clarifying Examples : Page 2 of this link