This should be called a "3-finger-tap" problem.
After I installed an app called Virtuoso from apple store, the screen was zoomed in somehow and I could not make it zoom out again. I thought this could be a bug of iPad, and got very excited.
Because the screen is zoomed in, and I can not reach the "slide to unlock" part of the screen to unlock it. Finally, by accidentally triply clicking the menu, the zoomed in mode disappears.
Taking a look at Setting->General->Accessibility, I found several interesting features iPad provided.
First of all, it can speak auto-text, which means it can read a book for us.
Secondly, when zoom mode is turned on. Double-tap with three fingers can zoom in/out the entire screen.
So this is not a bug, and I'm not lucky to find one. The problem must be caused by my tapping using 3 fingers when I was playing Piano.
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