If you press alt ingame your client will freeze, the client will still receive packets, but it won't do anything with it, rendering the player invulnerable until they re-enter their screen or press alt again.
A video showing this:
http://www.youtube.com/watch?v=5SeZ--8tRt0
for as far as I know this bug has been in minecraft since even before Alpha, in that time almost everyone knew of it, but now people are exploiting it again.
I can vaguely remember it had something to do with LWJGL, the server used to kick players for this, but it doesn't anymore.
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duplicates 1
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TL;DR: Considered be standard behavior for Windows applications with menu bar in any mode. Not considered standard behavior for programs with no (or different kind of) menu bar/system.
This is not forced by Windows on the application. Alt enables (or can enable) the menu (bar) on those apps which have it (and thus transfers activity focus to the menu, and the other parts of the app lose focus), but Minecraft does not have such menu.
As an example that is more relevant to Minecraft would be an MMORPG client played in windowed mode. Say, Anarchy Online or Eve-Online.
In the AO client I can start typing text in a chat window and/or move around, press alt or F10 and nothing happens; it still keeps working as before and no freezing or anything. Almost the same applies to Eve-online, except that in there F10 opens/closes its map view; still nothing freezes, let alone makes player invulnerable.
That is problem when using libraries: they behave like they should in given OS. Just now I had created program with only one text field and without menu bar. Pressed alt = lost focus. The only way to stop this is to move to another library which can be hard.
Duplicate of MC-5674, setting security to private.
It is the standart behavior for windows programs. Try opening some text editor, start to type, press alt (or F10), try to continue to type.
Note, that this behavior doesn't exist in fullscreen mode. Further, most Windows programs also doesn't have said behavior while in fullscreen.
TL;DR: Standart behavior for Windows programs in windowed mode.