Been wondering about copyright infringement for the giclee printing business. Such a fun gray area.

I print artwork reproductions and photographs, primarily for clients who sell their work at art fairs and in galleries.
I occasionally see a post on LL about some wanna-be photographer who ripped someone else's photo off, and put it in their portfolio. Just saw a case in the news about Walmart refusing to print someone's personal holiday photos, because they said they were too good not to have been done by a professional (who would hold the copyright.)
I have each customer sign an agreement that says they are the sole copyright holder, and they understand it is their responsibility to follow copyright law and are solely accountable for any copyright violations. As anyone with some legal experience knows, waivers don't trump law, so they only mean so much. If I was sued, I could sue my customer in turn, but it's not like they'd be able to pay.
So, what would happen if a photographer gave me an image, or an artist gave me a painting, and said it was theirs, when it was really someone else's work?
I'm not talking about a wink-wink nod-nod situation with some signed contract. That's not my game. If I ever believed someone might be asking me to violate a copyright, I'm going to need more convincing. I'm talking about an honest what-the-heck I thought that guy made that situation.
There's certainly cases of print shops being sued for copyright infringement. Kinko's was sued years ago for infringement on educational materials.
I know it's one of those one in a million situations that will never happen, but wondering if anyone's heard of any legal precedent in the US for this situation. The Kinko's case isn't exactly on topic. They had good reason to know there was infringement, since they were copying pages from different books to be sold to college students as course packets. Don't think they had a reasonable expectation that the person who brought them the book was the copyright holder.
Probably need to look into an insurance policy, but that's probably out of budget for now. (General liability policies don't cover this.) Wondering your thoughts.