Forgetting to include Form 4797 with your tax return can result in the IRS questioning your gain or loss calculations. In such cases, the IRS may require you to file an amended tax return to include the missing form and provide the necessary information. Failing to correct the omission can lead to potential penalties and interest charges. It's important to review the instructions for your specific tax return and ensure that you include all required forms and attachments, including Form 4797. If you realize you have omitted Form 4797 after filing your tax return, consult with a tax professional or refer to the IRS guidelines for guidance on filing an amended tax return and correcting the omission. They can provide assistance based on your specific situation and help you navigate the necessary steps to rectify the situation.