THIS BUG DOES NOT WORK ANYMORE!
Many people ask how is it possible that they loose their skills and not get any infos about this.
This due to a bug called "Stealth assault" or "blind death".
Description
Let's say we've got two gangs: A and B.
The Player is a member of gang B.
A leader of A sends a gang-invitation to the Player, but he (Player) does nothing with it -- doesn't accept nor decline.
Now A members are gang-assaulting the Player.
If A members are stronger but not twice as strong as B, they get skills, while B members loose their skills and not see any messages about it.
Solution
Do not invite unknown people to your gang...