C-x b *scratch* RET
C-x RET f latin-1 RET          ; buffer coding system = latin-1
C-u C-\ romanian-prefix RET    ; input method: romanian-prefix
,s                             ; character: ş (2362, #o4472, #x93a,
                               ;   U+015F)
<left> M-x quail-show-key RET  ; To input `ş', type ",s"
<right>
C-x RET f utf-8 RET            ; buffer coding system = utf-8
                               ; input method: the same as before
,s                             ; character: ş (331903, #o1210177,
                               ;   #x5107f, U+015F)
<left> M-x quail-show-key RET  ; ş can't be input by the current
                               ;   input method

Now, I understand that the buffer code for these characters is not the
same... but it is quite weird nonetheless to input a character with
the current input method, and afterwards be told that it "can't be
input by the current input method".

                    /L/e/k/t/u