Let's say I find out I could have taken a cap gains loss on the sale of real estate, oh, 18 years ago, or so, but I didn't, for whatever reason. It seems that attempting to correct that now would be incredibly complicated, but I'm wondering if it would even be possible. Is there some limit on how far back you can go to remedy a mistake on a federal return?

There is no deadline, but it depends;
There are a lot of different deadlines for a lot of different problems. aslongas you claim your tax refund, then, it must be made within 3 years from the due date of the return, or within 2 years from the date the payment was made, whichever is later. This law can bite a taxpayer that has discovered that he or she overpaid a tax in a prior year. You have just 3 years from the due date of the original return to file an amended return and get a refund of that prior paid tax. If you forgot to claim a deduction, you have until 3 years after the original due date of the return ;You have 7 years to claim a worthless security or a bad debt.