Thanks for the kind words rimma-sophia. guercini, you had better believe that I REALLY thought about just submitting the return because it did pass the review and told me "Great job!" too. My guess is that if they receive a return that has passed the error check by TT, they don't look too closely at the complicated issues. I didn't do it though because it was trying to give me such a huge deduction on Line 29 that I am not entitled to and even though that is in my favor and would save me thousands of dollars if they don't catch it, I just couldn't submit a return with a mistake that huge on it. I would think that if people have submitted returns with errors, after TT said no errors, and if the IRS has caught the errors, that TT would be forced to admit that their calculation is wrong and they would fix the problem but I'm not certain of that (see below). I believe that most people buy TT because they don't know that much about complicated tax situations like mine and they rely on TT to tell them what is right. If I was one of those people, I wouldn't have closely reviewed every entry on the forms and worksheets and would have just assumed it was accurate and, without realizing it was wrong, saved thousands of dollars.
Your last question is an excellent one. What WOULD happen if the error was caught after you submitted it when TT said it was correct? I can think of two possibilities, both of which would require you to send in the money owed, which is no harm done because you do technically owe it even if TT says that you don't. The first possibility is that the IRS just sends you a letter telling you how much you owe, you pay it, and that's the end of it and TT just continues making the error with their head in the sand. The second possibility is that TT will have to be confronted with the error because based on their guarantee of 100% accurate calculations, they will owe you (or the IRS) penalties and interest that accrued due to their mistake. Of course, if it goes the second way, that raises another question. When TT is forced to realize that they made a mistake, will they fix it or are they even able to fix it? With an issue this rare, I believe it is possible that even if "someone" at Intuit realizes that there is a mistake, they may decide that it is so rare that it would be cheaper for them to pay the interest for a few people each year than it would be to pay an entire team to do a major overhaul on the software. I am not a software guy but I assume it would be a massive project because of all of the iterations that are affected by so many different combinations things that even the "experts" don't deal with that often. The "experts" that I spoke with at TT about this topic all had many years of experience but every single one of them had to spend hours studying the IRS publications to brush up on the complicated stuff, just like I did. In the end, I tend to think that the mistake has never been caught by the IRS or on the rare occasion that it is, TT had no further involvement b/c I could see the IRS just waiving the penalties and interest due to the fact that it wasn't the taxpayer's fault. This would explain why the problems in this area persist year after year. As a side note that may interest you if you have read this much of my rambling, back in 2015 and 2016 when the PTCs first started, if you did not take any advanced PTCs and tried to fill in 0's for column C, TT would not allow you to go forward. You would get a pop up box that tells you that you must enter "an amount" in those boxes. Many taxpayers posted about this glitch and were finally told by TT to just put a penny (.01) in column C even though the 1095A had zeros for that column. Every single person who bought from the marketplace but did not take advanced credits had that problem and were forced to wrongly enter a penny for column C. I'll admit that entering a penny instead of zero is a pretty good "workaround" but just think about how many hours people anguished over this before finding this workaround and it wasn't fixed for the folowing year even though they definitely knew about it. At the very least, they should have put a popup on that page that said, "if your column C on the 1095A says 0.00, enter .01". IMO, they would never do that even though it would save hours of anguish because that would mean that they would have it in writing that they are instructing you to enter the wrong amount and that would be very bad publicity for them.
In sum, I agree with posters on other threads who have said that we buy TT hoping that it will make things easier for us and for most people, I'm sure that it does help a lot. Unfortunately, for those of us who have rare situations that take us to glitchy buggy areas, who are not willing to submit an inaccurate return, even if it is our favor and TT says it's accurate, it's a nightmare. I have seen other posts that say that we are all beta testers for their software, and you can see by the myriad upgrades and patches that frequently come out during the tax season that they have a very good point. If any TT people are reading this, I suggest that you make better use of beta testing your rare situation customers by changing your policy of having to start all over with a new "expert" on each call even though the previous "expert" has already spent several hours researching the issue. The new "expert" has to spend several redundant hours reviewing the notes from the prior "expert" and has to research the 98 pages of IRS documents all over again (Pub 974 and instructions for 8962). If there was a way that we could stay with the same "expert" until TT is forced to admit that there is an error in the software, then they could do a better job of catching glitches for rare situations. If they never admit that there IS a glitch, the can will just be kicked down the road time after time.