The point of extending the courtesy of trigger warnings isn't that you will, with 100% certainty, prevent anyone from entering a state of psychological distress. That's obviously not attainable.
But let's not let the perfect be the enemy of the good. There's really obvious stuff that instructors could warn about, but aren't. After I posted a similar sentiment on Facebook, a TA told me about their professor showing porn (featuring torture) in class. Who could have known that the majority-female class she was showing this to would contain some rape victims that left the classroom in tears?! Totally out of the blue, that.
Now, I don't expect anyone to know that aging wood with thin brown flaking paint is evocative of the assault I experienced; that would be ridiculous. But of course descriptions of assaults themselves are going to be evocative.
In short, there's really easy low-hanging fruit here, and I hope people aren't so turned off by the unattainable goals of safe spaces and 100% trigger coverage that they don't extend the courtesy of warning for really obvious, foreseeable triggers.