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Rivers can't be navigated, and it's now impossible to exit a boat without sending it flying back into the ocean, or into a wall. The control change has left boats fundamentally broken. I don't see how this bug can be marked as resolved, even if it was an intended feature.

This also affects me on Minecraft 1.4.7.

Minecraft 1.4.7
Ubuntu 12.10 (Linux) 64-bit
Java 1.7.0_09

The actual data is still there, because I can briefly see my changes in the affected chunk before it reverts to the default. My structures are visible in 3rd party map viewers (such as Minutor) as well. I'd be happy to provide further details if requested!

EDIT: In today's snapshot, 13w02a, the old changes I made to that chunk have now disappeared from Minutor. Any changes I make to the default chunk are now saved, but upon reloading my save the chunk reverts to the default chunk again. Items left in the affected chunk are saved, and can be collected from the affected area upon reloading the save.