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   (syntax-case exp (literal ...) clause ...)                syntax

   Each /literal/ must be an identifier.  Each /clause/ must take one of the
   following two forms:

   (pattern output-expression)
   (pattern fender output-expression)

So in your case (display ...) is considered a fender and the syntax-case is
well-formed (although perhaps not what you intended).  The fender is
evaluated, which prints output and returns an undefined value that is
interpreted as true.  So the output-expression is returned.

When you add another expression to the clause, the syntax-case is no longer
well-formed.  As the error message states, you have a "bad clause".

So just use (begin ...) or (let () ...) as you had in the original example.

HTH.

-d