Here is the list of common mistakes people make. Sometimes people get away with their mistakes, but that's no reason to make the same mistakes the next time, because the next time the same mistakes may cost you the game. Make it a habbit to quit bad habbits.
Zeek nev'r makez miztakez; all we'z miztakez Dez makez.
--Zeek n' Dez, Ogre-Mage of the Laughing Skull Clan
How many Peons are too many? The answer depends on how much resources are on the map, and how many players are in the game. Generally speaking, having too many Peons is not a concern. You need at least 32 Peons on most maps, and you want those 32 Peons quickly.
If you tell a Peon on his way to harvest lumber or gold (or a Peon harvesting lumber) to build a Town Hall, or if you tell a Peon on his way to harvest lumber (or a Peon harvesting lumber) to build a Lumber Mill, you get a free-trip of lumber or gold when the structure is finished (which means a free extra 100 or 125 wood, or an extra 100, 110, or 120 gold). This is a big advantage, especially on low resources. Make it a habbit to use the lumber bug for every Town Hall or Lumber Mill, even on high resources. It only takes 0.5 to 1.5 seconds to execute, and it gives you a significant advantage.
The most obvious example of this is building the Town Hall too far away from the Gold Mine. You want to build your Town Hall as close to the Gold Mine as you can, and you don't want to put buildings in the Peon's way to the 'Mine. The more common mistake is harvesting lumber too far from the 'Hall/'Mill or putting buildings in the Peon's way to harvest lumber. Place your 'Mill close to lumber not efficiently covered by your 'Hall.
People who use the keyboard give commands significantly faster than people who use the icons. Do not use the icons to give any command. Whatever you can do with the icons, you can do it faster with the keyboard. A less common mistake is not using the default commands the right mouse button gives. Using the right mouse button to give the default commands is significantly faster than using the keyboard or the icons. Make sure you know what default commands for each units are ('Sappers default to demolish on enemy units).
When engaging enemy land units in the open, use the Patrol command. If you use the move command, the enemy will get free shots against your units while they blindly follow your command to move to the spot. If you use the attack command on an enemy unit, the enemy will get free shots against your units while they blindly follow your command, moving around other enemy units in fatal attempts to get to that one enemy unit. Not using the Patrol command thus is the quickest way to lose land battles. Once the battle has joined, then start giving individual attack commands to gang up on adjacent enemy units.
If you have an ally, you need to work with him. The easiest way to kill an enemy is to gang up on him. This is especially true for Grunt rushes. Six Grunts will rip three Grunts to shreds with very little casualty, so make sure that you and your ally are the ones who are getting the six-on-three battles. Otherwise, you may be the one on the wrong end of the six-on-three.
Two Barracks do not cut it on most maps. You want at least one more Barracks than you can support. The same goes for other unit-producing buildings, including Mage Tower, Gnomish Inventor, and Shipyard. Having only one Barracks is the second easiest way to lose the game (the easiest being having no Barracks).
The most common example is upgrading the 'Hall too soon. Do not upgrade to Keep until you have at least 16-21 Peons. A more subtle mistake, which even good players make, is getting the second Weapon and Shield upgrades too soon. While you want to get the first Weapon and Shield upgrades as quickly as you can, the second upgrades require a little more thought. The rule of thumb is to get the second Weapon upgrade about the time you begin the Keep upgrade unless you are having serious early Grunt wars. A level 4 Ogre without the second Shield upgrade is as effective as a level 5 Ogre when engaging level 5 Grunts. While a level 5 Grunt is an even match for a level 1 Ogre, a level 3 Ogre will rip a level 5 Grunt to shreds. Don't upgrade to level 5 too soon without putting it to use.
Looking ahead on buildings is extremely important, either it's as small as a Farm, or as big as an expansion. Have the buildings up so that by the time you need them, you have them. Try to be a Farm ahead, and expand before your 'Mine goes below 15k. Go to the GameSpot's Insider's Strategy Guide and learn the construction time for each building.
How many Flying machines are too many? The answer once again depends on how much resources are on the map. Generally speaking, having too many Flying Machines is rarely a concern. The rule of thumb is that you need at least 10 Flying Machines. Simply having Flying Machines is not enough, you must put Flying Machines at critical places and move free Flying Machines around the map.
People play reactively either because they don't know where to attack, or because they are getting attacked. Playing reactively is usually a direct result of having insufficient reconnaissance. Don't just park Flying Machines outside your town and wait; send the Flying Machines to look for targets to attack. If you see overwhelming enemy forces heading for your expansion, don't keep your outnumbered forces in the expansion to die defending it, take an alternative route and hit his expansion while he works on yours. No man has won a war by dying for his country; he wins it by making his enemy's woman die for her country.
Lastly, this is the mistake that is so often the difference between victory and defeat. You know what to do, but you simply never get around to do it, either because you forget, or because you get distracted by little things like 18 Bloodlusted Ogre-Mages beating on your expansion. Often people forget to build more Peons. Always start a Peon before issuing commands to the just-built Peon. It's faster, and you don't run the risk of forgetting to keep building Peons (sometimes you get distracted while issuing commands to the just-built Peon). Each "ready to serve" message is a reminder to build another Peon. At other times people spend too much attention on Grunt rushing and they forget important things like upgrading and building Farms. Sometimes you get to Fortress and forget to build Temple of the Damned when you build the Altar of Storms. Periodically ask yourself if you are failing to execute, especially when you are attacking or being attacked. If you have an ally, remind him to execute as well. When you are incorporating new strategies into your game, write them down and do them. They best way to remember to execute is to make a habbit of the things you want to do; and you make a habbit of things by doing them.

War2 Mistakes / University of Utah / Ivory (sl7506@u.cc.utah.edu)
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