44 of the 50 states in the United States have a state income tax. This means that residents of those states must file a state income tax return every year at the same time they file their federal income tax return. State tax rules can vary from state to state so those who live in states where you are required to file state income tax returns have another set of rules to follow and more forms to fill out. This means there is more opportunity for errors to occur.
State taxes are controlled and collected by the state in question so the IRS is not involved. This means if you have trouble with your state taxes you have to deal with a whole new group of people. Like the IRS, states can audit you, demand payment, place liens on your property and garnish your wages. They can also hit you with penalties and interests.
In many instances states are less flexible because they are governed by specific rules or they have a smaller, overworked staff that is less likely to look closely at individual cases. It is just as importantly to properly file and keep up with your state taxes as it is your federal taxes. Dealing with the state is no easier than dealing with the IRS.