The code that actually determines whether a block is valid is not a small amout of code at all though - that's the entire problem! Whether a block is valid or not depends on subtle behavioural details of huge third-party libraries. The current issue resulted from a quirk of BDB and the developers still haven't managed to fully figure out when exactly it does cause a block to be treated as invalid. We also already knew that block validation inherits some of OpenSSL's quirks which aren't compatible with other, more spec-compliant crypto libraries. It's entirely likely that other code outside of the Bitcoin client also affects which blocks are valid.