Is it bad form to follow up a job interview with a further example to answer a question?

Some of the questions I ask may be intentionally tough and answering them with any real precision on the fly can be close to impossible. (Btw, if a question is that tough, at least in an interview I'm running, it's likely a test of how you react under pressure or whether you can acknowledge that you just don't know, not a hunt to pull a phenomenal response.)

I've had a handful of candidates do this, usually in the exact manner you suggest: as part of a thank you email. I tend to react positively. Some of the questions I ask may be intentionally tough and answering them with any real precision on the fly can be close to impossible. (Btw, if a question is that tough, at least in an interview I'm running, it's likely a test of how you react under pressure or whether you can acknowledge that you just don't know, not a hunt to pull a phenomenal response.) At any rate, someone who follows up later with a better answer (though I don't expect it) demonstrates a commendable level of thoughtfulness and indicates that they're serious enough about the job that they spent some time reflecting on their responses after they left the interview. The exception to this is a candidate who seemed ridiculously in their head or wordy during the interview. In that case, I look at it as confirmation that this is someone who isn't self-aware enough to know when to say when and may unduly second-guess themselves. But frankly if I left the interview with such strong reservations, well, that candidate isn't going any further regardless so the follow-up isn't the thing that cost them the job. I can't think of an instance when a solid candidate responded with an "addendum" and I held it against them.