What was the most recent change to the official client that broke block validity? There have been suggestions that various bitcoin protocol rules (max block size, mining reward policy, etc) could be changed in the future by everybody "agreeing to update their client".  The logistics of this are somewhat questionable.

Another question asked about the mechanics of such a change in the situation where people object to it.

I would like to know if there is any recent precedent for this that we could examine.  Specifically: have there ever been any changes to the official client version Y that would cause client version X to reject its blocks, where X &lt; Y?  If so, what is the most recent such case?

I suspect there were probably a bunch of changes like this back in the early days when only a few dozen people were using bitcoin.  It would be interesting to know if it's happened more recently.
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.