From a comment I added to the tracker that this change was attached to:
ZOOKEEPER-1136 causes a concurrency bug. Specifically:
1. Follower rejoins, gets snap from leader
2. Follower gets NEWLEADER message and takes a snapshot
3. Follower gets some additional tranactions forwarded from leader, applies these directly to data tree
4. Follower gets an UPTODATE message, does not take a snapshot
5. Follower starts following, writes some new transactions to its log, and is killed before it takes another snapshot
6. Follower restarts and gets a DIFF from the leader
The transactions that came in between NEWLEADER and UPTODATE are lost because they never go anywhere but the internal data tree, and if that tree isn't snapshotted and the follower restarts with only a DIFF, the follower will lose these transactions.
I think the proper thing to do is snapshot after UPTODATE, but I'm not sure why we changed this to snapshot after NEWLEADER instead. The wiki doesn't seem to explain that clearly.