=Sorry if my English isn't very good=

Sometimes chunks won't load until you mine a block at that place.

I walked on a not-loaded area, and normally it loads a chunk if you walk on it, but it didn't load, until I mined a block.

It sometimes naturally happens, but sometimes it happens by filling an area with air (attachment bug2a). Sometimes also blocks on fast-moving sticky pistons don't update very fast and smooth.