When you log in many chunks dont load around you, and most will refuse to load. With some of the chunks, you an update a block inside of that chunk to render it. This bug makes serveral more smaller bugs, like forever fly, fps drops, crashing the game, etc. Sorry, even though I try and get a screenshot I can never move my mouse in minecraft due to all the lag it causes.

(Note: this seems to only happen in singleplayer worlds.)