The different block chains is due to the new 0.11 clients starting to enforce the soft-forks from Bitcoin (BIP34, BIP65, etc).
These are enforced if a certain number of blocks are mined on the new wallet.
Since the majority seems to be on the new wallet, I suggest everyone upgrade to the new version to remain compatible.
To get the fork info on the new wallet you can type:
getblockchaininfo
I've been running 0.8.4.1 on a mac since April 2014, the last couple of days I get the message
'Warning: Displayed transactions may not be correct! You may need to upgrade, or other nodes may need to upgrade.'
As there is no new mac wallet available, what do I do?
Type in the above commands in Debug Window/Console?
Here's a Mac build I created from the 0.11.2.1 source. Give it a try: