I asked my usual suite of Schemes to evaluate (+ 1 (values 2 3)).
Racket, MIT, Gambit, Scheme48/scsh, Kawa, SISC, Chibi, Chez, SCM, Ikarus, Ypsilon, IronScheme, SigScheme all report errors, either of the form "Single value expected, multiple values received", or else a low-level error reflecting how multiple values are represented.
Gauche, Chicken, Bigloo, Guile, Larceny, Mosh, STklos reduce the multiple values to a single value. Of these, only Guile fails to convert zero values to the implementation-specific "unspecified" value.