I was playing with my friends running a server with 2048M and the following configuration:

http://pastebin.com/cNgiSm9Y

When we (3 players) walked ~250 blocks from spawn the chunks had load different and it wasn't updating blocks.
Example: We planted a tree (acacia) and we only had known it grow when we put a block next to it. Disconnecting and re-joining the server refresh as well.
Example 2: Some distant chunks doesn't load when approaching, but the blocks are there - it seems like the bug in snapshots of 1.8 (the markup is showing the block, but not the texture). Only when placing a block next to this buggy chunk it updates and load for the player (or reconnecting).
It happened when 3 players were playing. Sometimes occur "Java internal error", but I lost these messages :/