A Mass Stall is a common situation which occurs when a commander tries to build at a greater rate than his economy can keep up, resulting in the eventual depletion of his stored reserves. It is similar to an Energy Stall, except far less dangerous, and is more likely to occur during play. This most often occurs when building a large number of units at the same time, building emergency base defenses to prepare for an unexpected attack, or building expensive units such as T3 Heavy Artillery, Strategic Missile Launchers, Experimentals, and some high-end T3 structures. When a Mass Stall occurs, existing production is slowed as the incoming mass flow is distributed between units or structures. Also, no Air support unit will repair aircraft when insufficient mass is available.