This article covers a variety of common mistakes that can be done in the game, and will attempt to offer suggestions on how to avoid, resolve or work around them.
Saving over a save you do not want to overwrite
Occasionally you may find yourself mis-clicking on a save slot that you did not want to save over. If it's empty, then it's fine. If it's a save you do not want to overwrite, then this poses quite a problem as you cannot back out of the save once you get to the text entry for the file name.
Note that at this stage the file has not yet been saved. Once you hit enter, then the files will be overwritten. You can quickly recover your previous save by tabbing to your operating system and manually making a backup of the files or copying the contents into an empty slot. Once done, you can go back into the game and over-write the save.
This problem only affects the PC version of X-COM.
Selecting soldiers by Bravery
Early guides used to suggest all soldiers without high bravery be sacked. However, it turns out Bravery isn't that important. It reduces the chances a soldiers will lose morale (either from events or psi-attack) thus reducing the chances a soldier would panic or go berserk.
If X-COM is doing well (or has a high ranking leader with the team) Bravery would never come into effect. If not, than this is somewhat self-correcting since Bravery tends to improve after such an harrowing combat. A commander is better off selecting good shooters in order to avoid morale problems in the first place.
Building too many Radars
In spite of a misleading radar count in the base information screen, building more than one radar of each type is useless (at least unless UFO Extender program is used to patch UFO). Save your money and don't build more than one radar of each type.
An effect that often occurs through UFO doors or around corners with large number of aliens on the other side.
The common setup includes trying to breach a doorway with several soldiers. The first soldier is sent through the door, and is promptly killed. The next soldier is sent through the door, and is also executed in a similar fashion. Yet another soldier is sent through the door and meets the exact same fate. This process repeats until either the hapless soldiers are exhausted or the aliens are unable to react.
The root of this problem is lack of patience, and is easily solved by being patient and using time (and grenades) to assess the situation. Alternatively, just reverse the situation and have YOUR soldiers camp around the doorway and execute aliens that exit.
Watch my back!
The best way to lose a man in the field when its not X-COM's turn is to send a guy in with no one watching her tail-end. If nothing else, the screen won't immediately go dark around her. Even if the soldier's second can't save her life, the reaction shot might put down the killer or the LoS might allow the tracking of the enemy operative.
Neglect of battlefield awareness
The aliens generally do the following when fighting long-range battles: inch forward until enemy is spotted, shoot, break line of sight by taking cover or retreating out of sight range. This minimises X-COM retaliation during the X-COM turn as you have to waste TUs on moving closer or blasting away their cover first... that is, if you even know who just shot at you: if you're fired at from a direction no one is watching at the moment, you can twist and turn but you won't see the alien (because it's standing behind cover).
Nothing says you can't use the same tactic against them! If there is cover, use it. The AI doesn't realize it can intentionally destroy terrain obstacles and therefore won't shoot at you without line of sight. Of course, that means the alien in question will have plenty of TUs for reaction fire... but it's better than being killed during the alien turn because you have no TUs left for reaction fire of your own. That is, if the alien is in a position where you don't see it right after rounding the corner and therefore block reaction fire with the mutual surprise rule.