Recently I discovered (While pressing random keys to try to escape a creeper) that if you, for example, Hold down space bar (Jump) and then press "Alt" it will keep you jumping with out you pressing space bar.

What you do is first select a movement (Jump, walk, sprint etc)
Then you hold down that button and at the same time press Alt.
Minecraft should stop (It will look like it has frozen) but once you click, you will still be doing that action.
It will only last until you open your inventory, open a chest, press escape etc.
You could use this to climb up the thousands of stairs in your mineshaft, or just run long distances across a flat plain. Your choice. Have fun! Hope this has helped some people! =D This also works with the "Alt Gr" button.

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