fundamental error of interpretation of static steam dryness versus flowing steam quality in the coding of the simulator GWELL has been found, that means it does not correctly conserve energy or momentum. GWELL, or the energy and momentum subroutines from it, may have formed a basis for other geothermal wellbore simulator codes in existence, prompting this note on how to quickly check if your favorite geothermal wellbore simulator is actually conserving energy. The possible consequences of using a simulator that does not conserve momentum or energy as presumed, are also briefly explored. (C) 2014 Elsevier Ltd. All rights reserved.