At various levels skills and weapons can become intelligent. This means that they will no longer damage teammates. This also means that someone carrying an impostor power-up will be immune to them too! Most skills and weapons have four upgrade levels.
The Flame Thrower, Ice Cannon and Shockwave are all intelligent at level 3 and above. The Vortex is intelligent at level 2 and above.
Ray guns are intelligent at level 3 and above.
Jamming is intelligent starting at level 2.
Bullets are intelligent when they develop the heat-seeking capability at level 4.