If your interview isn't screening for the work you want somebody to do then it's not a good metric for finding successful employees.
This asymmetry exists regardless of how representative of real work the interview process is, short of a full-time engagement.
If he actually delivers good work, who cares if he cheats?
He already lied once. Why not lie more?
https://www.youtube.com/watch?v=iaSUOFleNRU
It's nearly 40 years old and it still makes me laugh every time.
Although that did happen, but I think that was the recruitment agencies: they'd debrief their candidates after their interviews, and the next time they sent someone in, they'd be prepped for the whiteboard design exercises we'd used previously. I ended up with a dozen different questions that I rotated, which seemed to be enough.
General interview questions were often prepped too. Our phone screen question list had to grow quite long before it seemed like the effort to cram/prep was too much trouble.
Firstly, all agencies prep their candidates to some extent. They know your stack, they know what skills and experience you're looking for, so they make sure the people they send in match the profile, and you know, just bone up on the important things a bit before the interview.
It's not a stretch to go from that to saying "and they'll ask about X and Y, so make sure you refresh your memory on that".
And then it's not far to outright "here's the list of stuff we know they've asked recently -- do your best!"
The worst offenders would get a grumpy phone call and sit out the next few rounds of recruitment, but ultimately you want the candidates, so ...
As we got into it, the candidate often asked me to repeat the question. I was happy to do so, but after a few times, I was already taking care to speak slowly and clearly, and it was still happening a lot.
Then I noticed a quiet murmuring sound after I'd asked a question. It was like .. a TV on in the next room or something. I ignored it at first, assuming it was someone else in the next room, but then it seemed like it was only happening after I'd asked a question, and before the candidate answered.
I was now pretty suspicious, but confused: what was going on? The candidate was flubbing the interview anyway -- their understanding was pretty shallow and they didn't appear at all confident in the material, so I wrapped up the interview, and showed them out.
As we were walking to the door, I stood beside them for the first time. And that's when I saw the earpiece: the candidate had a hearing aid-like earpiece in, with a wire running into their collar.
I was basically too surprised to do anything, and by this point we were at the elevator, so I just let them leave.
The way I figured it, afterwards, was that they had someone on a radio who they thought would help them with the interview. But either the radio was noisy or they just needed more time for the answer, so asking me to repeat the question was for the benefit of their remote assistant. If they'd been better at it, even had I seen the earpiece, I'd have assumed it was just a hearing aid. Fortunately, even with the remote assistance, they didn't pass.
Never happened again, but I can tell you I casually check everyone I've interviewed since for earpieces.