Radiation Budget Instrument (RBI) is a scanning radiometer that measures earth reflected solar radiance and thermal emission at the top-of-atmosphere. RBI has three radiance channels that cover 0.25-5μm, 5-100μm and 0.25-100μm spectral bands respectively. To ensure highly accurate measurement throughout mission life, RBI is equipped with two internal calibration targets to routinely calibrate the radiance channels on orbit. A highly stable Electrical Substitution Radiometer (ESR) based Visible Calibration Target (VCT) is used to calibrate RBI short wave and total channel; A 3- bounce specular trap blackbody Infrared Calibration Target (ICT) with high emissivity, High accuracy temperature measurement is used to calibrate the RBI long wave channel. Prior to launch, RBI will undergo a comprehensive ground calibration campaign in a thermal vacuum chamber developed for RBI at the Space Dynamics Laboratory (SDL). A set of calibration targets developed by SDL, including short wave radiance source (SWRS), long wave infrared calibration source (LWIRCS), and a space view simulator (SVS) were used for RBI ground calibration. The plan is to characterize RBI absolute radiance measurement accuracy and repeatability, tie internal calibration targets to ground calibration, to carry the ground calibration to orbit. In fall 2017, the RBI Engineering Development Unit (EDU) went through the ground calibration campaign, as the pathfinder for flight unit. A large discrepancy was observed between the SDL target based calibration and RBI internal target based calibration. In this paper, we describe the discrepancy observed, the root cause analysis, and some lessons learned.