Great topic - and I already agree with all of the answers. MFS is often a red flag.
Split dependent benefits for divorced clients have also brought me bad "luck".
Bankrupt clients = bad.
Under-withholders - with the exception of some who don't pay ES tax but have it put aside due to the low interest rate environment.
Those who start by complaining about their previous preparer (unless it was for unresponsiveness) or admit they were fired.
"I usually use turbo tax but this year I am showing a large balance." This is the worst.
Those who want free(!) tax advice or your time when they are not yet clients. These never work out well.
Normally, I would agree that any mention of the words "turbo tax" has not worked out well on many, many, many levels.
But let me play devil's advocate on that one for a moment. I have had self-preparing new inquiries come to me because they sold a rental, or installed solar (form is complicated in Hawaii) and I charged them a ton, knowing it would be one and done.
But some of these clients have stayed with me saying, "it's just so convenient and I sleep better, I just assume you do it every year even if it's simple".
Not that I would ever discriminate based on gross income - but it seems like the higher earners fall into that category or just make better clients in general. I'm just mentioning this point for a friend - I would never make that a determining factor.
It's often the middle earners that turn out to be non-compliant or "schemers". Though I have seen plenty of rich and non-compliant filers too.
Let me turn the topic a bit into who makes good clients....
I've had good luck with military clients, such as physicians, and those who become DOD contractors. Sure, the "states" are a PITA, but they retire from service and many become s corps, etc, and they are great to deal with. Military pilots are good too and they leave the military and become airline pilots and property owners.
I also like new business clients who have no problem paying for a consultation.