Stupid things I have done...

[Extraordinarius]

After separating the units for a fairly large combat into their platoons and companies, forgetting to deploy them in the setup phase. A company of infantry crammed as far back in the corner as possible, especially when it is the friendly corner is furthest from where you planned to attack (and placed the rest of your force) does you no good, espcecially when the enemy have a valley with a wonderful field of fire between you and the rest of your troops. I have done this more than once.