In the Illinois 1040-ES, estimated tax payments have penalties assigned to them in different amounts depending on whether they were late and how late they were. On the form, it states:

"We will apply each payment to the earliest due date until that liability is paid, unless you provide specific instructions to apply it to another period."
https://www2.illinois.gov/rev/forms/incometax/Documents/currentyear/individual/IL-1040-ES.pdf

There is a clear benefit to apply all payments to the quarter closest to the date that the payment was made. Consider the example where the total amount due was $4000 ($1000 per quarter) and the amount of estimated tax paid was $3000 on 1/15/2020, with no earlier installments being made. TurboTax applies $1000 to each of the first three quarters, each with a late payment penalty of $100, and a late payment penalty of $100 for the fourth quarter for a total of $400 in late payments. This is what the Illinois form 1040-ES specifies by default. However, it would be strictly better for the taxpayer to apply them backward, which would result in a total late payment penalties of $300, as the fourth quarter would not be late.

The relevant form in this case is Form IL-2210: Computation of Penalties for Individuals, which appears in TurboTax but is not editable.

There are two solutions to this which could be implemented by TurboTax:
-One solution for TurboTax would be to allow for taxpayers to specify which quarters their payments go to.
-A simpler solution is to automatically apply these (insufficient) estimated payments backward, reducing late penalties for taxpayers. The IL-1040-ES instructions seem to allow this, and doing so in reverse is always better for taxpayers, though it is not the default. All that needs to be done is the provision of specific instructions.