<sigh> Or when the Oracle SCN advances too quickly, causing random systems to refuse connections or crash outright. Just because software is large, supported, and mature doesn't mean it is free of serious design flaws.
[Edit:] Don't get me wrong, Oracle's DBs are serious business; an incredible feat of engineering. If I could afford them I'd probably use them more often. But everything in this business is a cost tradeoff--in licenses, in dev time, in risk.