Some aspect of your base is unreliable. Maybe your vehicles break down, or electrical storms regularly disrupt communications, or the alien gadget is especially problematic.

Pick one of your Good Base Traits. That Trait is now unreliable. Sometimes, it just isn’t available. For example, an Unreliable Mobile Base might only be able to move under certain conditions. An Unreliable Secure Base might be mostly secure, but occasionally gaps open up in the security perimeter.