Laffer curve. The Laffer curve posits that tax revenue will first rise and then eventually fall as tax rates increase. This result is due to the effect that income tax has on the incentive to work.
As income tax initially rises, tax revenue increases. At T* the government have set the income tax rates that maximize tax revenue. After T*, any further increase in income tax means a lot of workers will quit their jobs because their after-tax wage is too low, they would rather go on benefits. Workers may also have an incentive to evade tax if it is too high. So tax revenue begins to fall. At a 100% rate of tax, nobody works so tax revenue is zero.
The Laffer curve suggests that governments may need to decrease tax rates to increase tax revenue.