Latching onto one salient property as a reliable guide. From Piaget's classic investigations on conservation of volume, research has identified a tendency to fix on one sensible feature of a situation to reason about a quantity. Children in Piagets investigations latched onto the height of the fluid in a container, using this as a proxy for volume.
Similar salient features of situations or processes are common in powering a home. Careful thinking — often reimagining am the situation or process — will be needed to avoid this tripwire.