I was able to derive it on the fly fast enough to make a StackOverflow answer on...

I was able to derive it on the fly fast enough to make a StackOverflow answer once. I had never heard of or used the technique before, and certainly didn't know what it was called.

I'd probably wiff it in an interview too. For some reason my brain doesn't work as well in an interview setting. Thankfully I haven't had to worry about that too many times.