This slip-up takes place in the scene where Ripley is on the drop-ship gearing up to go into the processing tower sub-level to rescue Newt. She first takes the flamethrower off the weapons rack, but then lays down the M41-A Pulse Rifle. She then reaches for the Pulse Rifle on the weapons rack, but lays down the flamethrower. It's a simple mistake of editing that was not noticed and has never been corrected. (At least not that I'm aware of.)