I describe the problem from 'emacs -Q':
scheme-mode.el isn't able to correctly parse the file:
racket-5.0.2/collects/drracket/tool-lib.rkt
When I open the file, go to position (79,0)
and execute `forward-sexp' I get
Scan error: "Unbalanced parentheses", 2852, 65105

The expected behaviour is for the point to be moved at the end
of the file behind the last closing parenthesis to position (1663,85).
When I move the point at the end of the file behind the last closing
parenthesis and do `backward-sexp' the point is moved to line
the point is moved to position (152,19) instead to (79,0), where
the next character is an opening curly brace.

This error occurs neither under fundamental-mode nor under lisp-mode.
I attach the problematic file to this report.

Thank you very much,
Alexander Haeckel