Well, i know the disappearing items from inventory has been mentioned but i'm not sure it has been mentioned that it happens with companions to.
I gave Boone the cowboy repeater and went to do the ghoul/rocket mission. In the start all went fine suddenly i noticed he was using a 10mm i also gave him.
I went into his inventory and the cowboy rifle was gone. I went back up the road to see if it was dropped but it was just plain gone.
This happend within a timeframe of 10-15 mins.