Rather interesting that a strict reading of the Illinois Constitution indicates that it does not require a balanced budget. Rather, what the appropriate section says is that:
The General Assembly by law shall make appropriations for all expenditures of public funds by the State. Appropriations for a fiscal year shall not exceed funds estimated by the General Assembly to be available during that year.
Ergo, the constitution only requires that the legislature keep spending within funding estimated available for the year. It does not require appropriations to be adjusted for actual funding or to address outstanding bills left over from prior years when actual revenues fell short of estimates.