[plt-scheme] [r6rs mode] when are the irritants appended to the message?

Using plt-r6rs version 4.2.4 the program:

(import (rnrs)
  (rnrs eval))
(error #f "ciao" 123)

prints:

ciao
 irritants:
  123

in which the string:

"\n irritants\n  123"

is appended to the  string in the &message condition object.
The program:

(import (rnrs)
  (rnrs eval))

(display (guard (E (else (condition-message E)))
	   (eval '(syntax-violation #f "ciao" 123)
		 (environment '(rnrs)))))
(newline)

prints:

ciao

which means that the message condition object is left alone.
Fine.

  I  have  a program  using  EVAL  in  which the  result  of
CONDITION-MESSAGE has the  irritants appended; I cannot post
the program  (it is too long),  and I have not  been able to
isolate the  behaviour with a  shorter program.  I  may find
the  problem if  someone  is so  kind  to tell  me when  the
irritants are appended.

TIA
-- 
Marco Maggi