I remember a discussion with Chuck about this several years ago.  Not going to try to find it now though.  I think AM was involved also.

My understanding is those are circumstances such as the business income limitation as Monkey mentioned, or maybe an employee home office that doesn't get over the 2% hurdle (or I guess any employee home office post-TCJA).

Fortunately, we have a fairly relaxed 3115 "get out of jail free" card.  Yes, it's more paperwork but that's the cost of prior year DIY returns (and I'm not afraid to tell clients that).  Nothing is lost, it's just a chore to get it back.