When a redstone torch is placed on the side of a block, it's powered. However, when redstone dust is placed on top of the block the torch is on, and a block is placed directly above the torch, the torch is flash, then turn off.
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http://www.minecraftwiki.net/wiki/Redstone_torch
Redstone torches will burn out when switched between the on and off states too often (8 or more off-on-cycles within 60 ticks (3 seconds)). This can happen if the player clicks a lever quickly and repeatedly with the lever being hooked up to a redstone torch, or when the player creates an infinite feedback loop by inadvertently or purposely wiring a redstone torch back into itself (a one clock). This will cause the torch to repeatedly apply and remove current, allowing the torch to come back on and once again apply current to itself. This causes the torch and any mechanisms wired to it to rapidly activate and deactivate repeatedly and could crash the game depending on the complexity of the mechanism.
This is likely why Notch programmed the torches to burn out. When burnt out the torch will issue a puff of smoke and a hiss similar to an extinguished fire. Then it will ignore any events which would normally turn on the torch until enough time has been elapsed so that there are 7 or fewer off-on-cycles within the last 60 world ticks. After that a new event of unpowering the blocks adjacent to the torch is necessary to turn it on again (the current state of the blocks is ignored, it requires a new on-off-edge or a general kind of block update). There is no limit on how often a single redstone torch can burn out.
Intentional. Four torches hooked together would work.