I'm entirely serious. To cite one of the most famous examples, Facebook ran a sort of A/B test where its algorithm was adjusted to attempt to make users happy or sad.

There's a fairly strong case to be made that intentionally making a large number of people sad just to see if you can is unethical. There's a somewhat weaker case to be made that manipulating the happy group was also an unethical distortion of their reality. I fail to see an ethical problem with the fact that it was an A/B comparison. Instead, A and probably B would be unethical to attempt under any conditions without consent.