The GUITAR error occurs when too many items aren't picked up off the ground in an activity, such as Gambit. The best way to ensure that this error doesn't occur is to make sure that items are being picked up off of the ground in those activities.
Items that can fall to the ground include:
- Orbs of Power
- Engrams (Rare, Legendary, Powerful, etc.)
- Ammo (Primary, Special, Heavy)
- Destination Materials