A Health and Safety Executive (HSE) investigation into a gasoline overfill of a carbon adsorption vapor recovery unit (VRU) revealed concerns with the VRU’s design. The overfill prevention system was NOT INDEPENDENT of the basic process control system (BPCS). When the BPCS failed, the overfill prevention system also failed. This resulted in loss of containment, and risked a significant fire and explosion, as well as extensive environmental damage.