Since 1.6.1 chunk loading, especially on servers has messed up.
It will load some chunks, miss out others, or be really lag for some.
The screenshot shows it.
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Also doesn't work properly in windows 8
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Single-player also seems affected.
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Duplicate of MC-129, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being closed as duplicate.