It is scary to think about 1) how much control CBP officers have over the fates of people entering the country especially on non-citizen statuses, and 2) how little they know about the rules affecting the latter.

E.g. I've heard of CBP officers not knowing that Canadians don't need to get a visa to enter the US. Also heard of CBP officers not knowing what "Massachusetts Institute of Technology" was on a student's F-1, and had to ask around to confirm that it was a legitimate school. And many more.