The 84-35 RP is a great get out of jail free card but if you find your clients did not all file timely returns (no extension filed; filed late) then your last two remedies are the First Time Abatement option (Google FTA, IRS) or just simply reasonable cause, which is a valid option if this is their first experience with a Partnership structure.

Also, I had a client structure a large (over $10MM) 1031 Exchange last year. I urged them to go to a more competent preparer as I know my limitations and 1031s of that size are not in my wheelhouse. The preparer of the entity return; someone well versed in tax law, was not aware of the change in due dates from April to March this year for Partnerships. This is another form of reasonable cause if the Rev Proc is not available to your clients and you don't want to use up the FTA.

I had a handful of clients get tripped up on this one, mostly new clients.