Yes. Go to System > Tools > Compilation and click the Disable button.
Take 2: Files in app/code/core/
can be overridden in app/code/community/
and likewise by app/code/local
. If you edit a file in core
that is present in one of the other directories then your edit has no effect.
Take 3: If you have APC or XCache installed on your server then it might not have noticed a changing file. Restart the apache or php-cgi process.