user307: I only get to work on my arduino every few weeks, so sometimes and get interrupted a lot by small children, so if I completely forget what is on the firmware, then I unplug the pins (because I haven't got anything soldered on there) should I be 100% certain I'm not going to fry my board?

user220: 100% certain? No. 99.9999[Repeating]%? Yes
Here's a couple problems you could run into:

Using pins 0 and 1 when connected to USB (wouldn't damage it, the USB chip if on a board with a separate USB chip would probably ignore the "corrupted" data.)
You never mentioned your board. If it's a Uno and you modified your USB firmware, there might be some issues reflashing your board if you forget that you flashed it with a keyboard firmware. It won't damage it, but you may think it's bricked if you forget to reflash it.
If you only disconnect a couple pins and leave others in, it may give wierd results and fry the Arduino or whatever is partially attached
If you make any physical modifications to the board

The second to last one is the one you really have to worry about. I have seen previous discussions about destroying a board with a sketch, but nothing has been suggested.
Lastly, there a chance you can damage it with setting the fuses on the chip(s) on the board.

user28: It's generally a good idea not to try to take input from a pin configured as pinMode(OUTPUT) and vice versa. In addition, I prefer not to take input from tristated pins. Both of these cases are harmless, but it's good practice not to do so, especially since they can cause issues when coupled with some hardware.
In general, Arduinos are protected from software issues. You can't burnout an Arduino with just faulty software.
There are, however, many ways to fry an Arduino via hardware.

user316: Arduinos are far from bulletproof but with nothing connected (apart from a power source of course) I doubt it will come to harm. If in doubt, upload Blink to it.
There are rugged Arduino-compatibles. One of them has a rather obvious name

user83: Although the controllers on an Arduino are pretty well protected, they can still be damaged by ESD (Electro Static Discharge). The same effect that makes a crispy sound when you pull off your woolen sweater in the winter, or after walking with rubber shoes on a carpet and you touch someone else or a metal and grounded object and you feel a pick. Static electricity can damage the controller. It may get completely broken, or it may function 'most of the time' or lifetime may be shortened.
An Arduino is also easily killed with an piezo gas lighter, just make the spark near the copper traces that feed directly in the controller. This is largely the same effect as above. 
Again, the controllers themselves are pretty well protected, don't worry too much about them. But with kids running around (on their rubber shoes) better make sure they don't accidentally draw a spark from their little finger to your Arduino. The protective metallic bag is best, but putting it away in a cupboard or box should do the trick equally well.
As a general rule of thumb, the faster the electronics, the more sensitive it is for ESD. A Due is probably more sensitive than a UNO or a MEGA. But with normal, avoiding static electricity, use you should be OK.

307: <p>I only get to work on my arduino every few weeks, so sometimes and get interrupted a lot by small children, so if I completely forget what is on the firmware, then I unplug the pins (because I haven't got anything soldered on there) should I be 100% certain I'm not going to fry my board?</p>
 220: <p><strong>100% certain? No. 99.9999[Repeating]%? Yes</strong></p>

<p>Here's a couple problems you could run into:</p>

<ul>
<li>Using pins <code>0</code> and <code>1</code> when connected to USB (wouldn't damage it, the USB chip if on a board with a separate USB chip would probably ignore the "corrupted" data.)</li>
<li>You never mentioned your board. If it's a Uno and you modified your USB firmware, there might be some issues reflashing your board if you forget that you flashed it with a keyboard firmware. <strong>It won't damage it, but you may think it's bricked if you forget to reflash it.</strong></li>
<li>If you only disconnect a couple pins and leave others in, it may give wierd results and fry the Arduino or whatever is partially attached</li>
<li>If you make any physical modifications to the board</li>
</ul>

<p>The second to last one is the one you really have to worry about. I have seen previous discussions about destroying a board with a sketch, but nothing has been suggested.</p>

<p>Lastly, there a chance you can damage it with setting the fuses on the chip(s) on the board.</p>
 28: <p>It's generally a good idea not to try to take input from a pin configured as <code>pinMode(OUTPUT)</code> and vice versa. In addition, I prefer not to take input from tristated pins. Both of these cases are harmless, but it's good practice not to do so, especially since they <em>can</em> cause issues when coupled with some hardware.</p>

<p>In general, Arduinos are protected from software issues. You can't burnout an Arduino with just faulty software.</p>

<p>There are, however, <a href="https://electronics.stackexchange.com/a/67151/8144">many ways to fry an Arduino via hardware.</a></p>
 316: <p>Arduinos are far from bulletproof but with nothing connected (apart from a power source of course) I doubt it will come to harm. If in doubt, upload Blink to it.</p>

<p>There are rugged Arduino-compatibles. One of them has a rather <a href="http://ruggedcircuits.com/html/ruggeduino.html" rel="nofollow">obvious name</a></p>
 83: <p>Although the controllers on an Arduino are pretty well protected, they can still be damaged by ESD (Electro Static Discharge). The same effect that makes a crispy sound when you pull off your woolen sweater in the winter, or after walking with rubber shoes on a carpet and you touch someone else or a metal and grounded object and you feel a pick. Static electricity can damage the controller. It may get completely broken, or it may function 'most of the time' or lifetime may be shortened.</p>

<p>An Arduino is also easily killed with an piezo gas lighter, just make the spark near the copper traces that feed directly in the controller. This is largely the same effect as above. </p>

<p>Again, the controllers themselves are pretty well protected, don't worry too much about them. But with kids running around (on their rubber shoes) better make sure they don't accidentally draw a spark from their little finger to your Arduino. The protective metallic bag is best, but putting it away in a cupboard or box should do the trick equally well.</p>

<p>As a general rule of thumb, the faster the electronics, the more sensitive it is for ESD. A Due is probably more sensitive than a UNO or a MEGA. But with normal, avoiding static electricity, use you should be OK.</p>
