The penalty for late filing of IRS payment plan form (Form 9465) depends on the specific circumstances. If you fail to file the form on time, the IRS may charge you a late-filing penalty, which is usually 5% of the unpaid taxes for each month or part of a month that your return is late, up to a maximum of 25% of your unpaid taxes. Additionally, if you don't pay the full amount owed by the original due date, the IRS may impose a failure-to-pay penalty, which is typically 0.5% of the unpaid taxes for each month or part of a month the taxes go unpaid, also capping at a maximum of 25% of your unpaid taxes. It's important to note that penalties and interest charges can vary based on individual circumstances, such as late payment history or the amount owed. If you're facing difficulty in meeting your tax obligations, it's generally recommended to contact the IRS as soon as possible to discuss payment options and potentially avoid or minimize penalties.