The Tower Bug
The tower bug wasn't discovered until years into Warcraft's life. This bug is a major flaw in the game and can be exploited for strategic purposes in all kinds of situations. In fact, if you don't know how the bug works, you might think someone was hacking when the enemy towers seem to shoot further than yours.
A range of 6
What the heck is this then?
Here is a simple screenshot to introduce the tower bug. The enemy tower is on the left, and my tower is on the right. As you can see, the towers are shooting at each other. The red arrow shows you that my peon SHOULD be in range of the enemy tower. And since a tower will always shoot at units before buildings, why is my peon not getting killed? The peon is located 6 squares to the right of the enemy tower, so it should be getting hit, right? My peon is safe in this picture because of the tower bug. Something really strange happens to a tower when there is an upgraded enemy tower within range.
Now, when a tower does NOT have another enemy tower within range, the tower seems to work fine. It will shoot units or buildings located 6 squares in any direction. However, when there is an upgraded enemy tower within range, the way towers work change. What happens is both towers (yours and the enemies) now shoot out of the top left corner. I've represented this by the green color in the picture. So now go back up and take a look at the previous example. My peon is not being killed in that example because the enemy tower is shotting from its' top left 'square'. That means that the numbered arrow at the bottom is actually wrong. The enemy tower is shooting from the top left, so the first range will only be making it to the right side of the tower. The enemy tower has lost a 1 range because of this.
'Up' and 'Left' have more range
To the left...
The first picture shows you the range that both towers have. Since the bug is in effect (an upgraded enemy tower within range), both towers are shooting their arrows from the top left corner (shown by the green box). You can see that this causes the tower on the left to actually lose 1 range compared to the tower on the right. The second picture is simply the results. There is no way that the purple player can repair that tower without losing peons. The player on the right is completely safe, and will never be hit by the enemy tower.
Some other things to keep in mind
This works for cannon towers too. Because cannon towers have a range of 7, you have to add 1 range to everything. Place your cannon tower 4 spaces away instead of 3. I did not cover this because you will rarely use this. Maybe once in 2500 games, and it will be when you and your enemy moved to the same starting position (eg. mid-left on garden of war).
Watch for this all the time. This is used alot, and can win and lose you games. If an enemy is going to kill your tower with this bug, you have to do something to prevent them from towering into you endlessly. Just remember, 3 spaces in between the towers means the tower bug, and that the tower bug gives a tower more range up, left, or any combination of the 2.