These both cause an error (in both 5.26.0, 5.25.1, and gcl,sbcl)
is("zebra" > 0);
every(lambda([x],is(x>0)), [1,2,"zebra"]);
with sbcl:
Maxima encountered a Lisp error:
The value "zebra" is not of type LIST.
gcl:
Error in MACSYMA-TOP-LEVEL [or a callee]:
Caught fatal error [memory may be damaged]
-- John Lapeyre