Experiments on forced ignition of extremely fuel-rich n-butane-oxygen mixture with the equivalence ratio of 23 in the standard 20-liter spherical vessel at elevated initial pressure (4.1 bar) and temperature (500 K) reveal the nonmonotonic influence of the forced ignition delay time on the maximum explosion pressure and the maximum rate of pressure rise. The objective of the study reported herein is better understanding of test mixture oxidation and combustion in the 20-liter explosion vessel by means of mathematical modeling of the accompanying phenomena. It is shown that several temporally and spatially coupled phenomena could take place simultaneously in the experiments. These are mixing caused by oxygen injection to n-butane, forced ignition, flame propagation, preflame oxidation, heat transfer, and natural convection. Based on the CFD simulations of the mixing process and natural convection of the ignition kernel, as well as on the analysis of the detailed reaction mechanism of n-butane oxidation, laminar flame propagation, and self-ignition, possible explanations for the phenomena observed in the experiments have been suggested. The results of the study indicate that apparently inflammable mixtures can nevertheless become hazardous depending on the mixture preparation procedure and forced ignition timing.