The MySQL 5.0.26 released 3 Oct 2006 contains an embarrassing ABI breakage. We're fixing this bug right now, and making a new build including the first binaries which will carry the name "MySQL Community Server" and "MySQL Enterprise Server" (which obviously did not exist on 3 Oct 2006). We'll notify both our community and our customers of the new builds when they're available.
Sorry about the trouble. We're also improving our internal QA and build processes, in order to avoid future, similar problems.