Once in an interview I asked someone to go to the whiteboard for a coding exercise and her body language showed an enthusiasm and fearlessness that in hindsight I realized I'd never seen before. She practically sprung up out of the chair.
Most people, even the good ones, show a little hesitance when starting. Which isn't really necessary, most people do fine. I'm not trying to get them to fail, I'm trying to get them to succeed. I want to see if they're smart and understand the problem and the direction of a solution. Not if they miss any semicolons or don't recall some arcane data structure.
She was one of the best hires I made. Coding interviews can also measure attitude and confidence.
Once in a coding interview, someone asked the candidate to go to the whiteboard and the poor guy was so flustered he couldn’t remember how many bits were in a byte.
He was a perfectly intelligent programmer and a fine person. I have no doubt at all he understood the details of bits and bytes. But, that group interview session did not sufficiently manage the stress level of the process. And, so we probably missed out on a perfectly good hire.
That and similar experiences in other group interviews are why my 1:1 interviews are structured around keeping the stress level low and preventing the candidate from freezing up.
Blaming stress only goes so far. If you're frozen up about how many bits in a byte, this knowledge really isn't implanted very deeply. Anyone can figure out the powers of 2 by doing tedious addition in their head, but if you spend a lot of time programming you tend to have the first 10 or so memorized.
In music performance, stress makes everything harder, but that's not an excuse - you need to practice until the motions are so deeply ingrained that it's practically part of your autonomic nervous system. Coding isn't so different.
I also go out of my way to make the candidate feel they're not being judged harshly or pedantically.
But just like interviews, jobs can also be stressful despite the best intentions to avoid toxicity. Only being able to perform under comfortable conditions does matter.