If a macroexpansion communicates with other parts of the system using eg. structures or instances, expanding nested macros doesn't work in Slime:
...assuming you have CFFI installed and loaded...
(cffi:defcallback foo :void ((data :string)) (error data))
C-RET expands it to
(PROGN (CFFI-SYS:%DEFCALLBACK FOO :VOID (DATA) (:POINTER) (LET ((DATA (CFFI:TRANSLATE-FROM-FOREIGN DATA
#<CFFI::FOREIGN-STRING-TYPE :UTF-8>))) (BLOCK FOO (ERROR DATA))) :CALLING-CONVENTION :CDECL) 'FOO)
where the %DEFCALLBACK is also a macro. Trying to re-expand that using C-RET breaks when it tries to READ #<.
It would be nice if (a) the second expansion worked (2) the #<...> objects in the buffer were presentations.
Cheers,
-- Nikodemus