For example, if they were convinced they were a frog, and protested loudly to anyone who tried to convince them otherwise, and seemed pretty damn happy thinking they're a frog, would you just let them be or would you feel compelled to correct them?
I would correct them, if it's something objective that can be proven and measured undeniably, and is not dynamic.
I would be more selective, careful and hesitant otherwise, which is most of the cases, when it's about a person. Personalities, and even bodies, are dynamic systems, which undergo change, and are influenced very much by what they are told. Objective truth and measurement is then a deceptive illusion. Like if you measure a sinusoid at just one moment of time, and claim it's constant, at that same value. Truth is not just truth, it depends on settings, measures, time, reference, frames. It tends to be so
relative, that it requires very special approaches, in order to reduce personal bias.
In general, I tend to have this flaw of being extremely honest, to the point of hurting people. It's because I don't perceive many of the things I say as negative. Most social ideals are derogatory, because they assume impossible goals, and leave most people upset. I do not subscribe to those ideals usually, and don't view deviation from them as negative.