<p>We introduce a database of 20 accurate cycloreversion barrier heights of 5-membered heterocyclic rings (to be known as the CRBH20 database). In these reactions, dioxazole and oxathiazole rings are fragmented to form isocyanates, isothiocyanates, and carbonyls. The reference reaction barrier heights are obtained by means of the high-level, ab initio W1-F12 and W1w thermochemical protocols. We evaluate the performance of 65 contemporary density functional theory (DFT) and double-hybrid DFT (DHDFT) procedures. The CRBH20 database represents an extremely challenging test for these methods. Most of the conventional DFT functionals (74%) result in root-mean-square deviations (RMSDs) between 10 and 81 kJ mol^-1. The rest of the DFT functionals attain RMSDs = 5-10 kJ mol^-1. Of the 12 tested DHDFT functionals, only five result in RMSDs < 10 kJ mol^-1. The CRBH20 dataset also proves to be a surprisingly challenging target for composite and standard ab initio procedures.</p>