-) I am new to the ADD world, and have some problems with weapon proficiency.
I understood the specialization mechanism and how it works, but what about penalties ?
The manual stats that if you use a weapon you are not specialized at, you'll suffer a penalty (-1 to hit and -1 damage).
It seem not to be true, since with my wizard when I use an 'untrained' weapon I suffer a -5 penalty in THAC0.
How is it possible ?

Is it possible that you are comparing it to a weapon that you are proficient with and get other bonuses?

No,
I get the -5 penalty w.r.t. to no weapon used configuration, i.e. the penalty is obtained by simply adding the weapon in the quick weapon slot