Missing whitespace between alphanumeric characters and quotes leads to a error "unbound variable".
Example to reproduce:
(define a 5)
(eval '(list "abc" a"xyz"))
Note the missing space between a and "xyz".
Tested against CVS HEAD (scheme.c at revision 1.35).
r5rs definition is strange here ("Tokens which require implicit termination (identifiers, numbers, characters, and dot) may be terminated by any <delimiter> .."), but the quote is explicitely not in the set of valid identifier characters. So the error "unbound variable" is definitely not correct