SBCL is mostly an implementation of the ANSI standard for Common Lisp. However, there's some important behavior which extends or clarifies the standard, and various behavior which outright violates the standard.
Essentially every type of non-conformance is considered a bug. (The exceptions involve internal inconsistencies in the standard.) In SBCL 0.7.6, the master record of known bugs is in the BUGS file in the distribution. Some highlight information about bugs may also be found in the manual page. The recommended way to report bugs is through the sbcl-help or sbcl-devel mailings lists.