What bug makes you to recommend upgrading most frequently ? For me it is this bug which makes it quite painful to automate various replication tasks.
It is not the most critical bug by far but this makes it worse – critical bugs would usually cause upgrades already or were worked around while such stuff as causing things like “sometimes my slave clone script does not work” may hang on for years.
There’s one fixed in the late 5.0.x series that breaks INSERT … ON DUPLICATE KEY UPDATE when the query cache is enabled — the cache doesn’t get cleared, so it keeps returning the unupdated values. Absolutely infuriating!