What was the most recent change to the official client that broke block validity?
The last changes I'm aware of that would cause a blockchain split were in August 2010, after the overflow bug; see, for example, https://github.com/bitcoin/bitcoin/commit/4bd188c4383d6e614e18f79dc337fbabe8464c82

I don't think there have been any changes to the block or transaction validity rules since, although it is very possible I'm forgetting something.