Early 2010's, Wall St software tech vendor: I had a candidate show up for an interview, in person, go through the preliminaries and then sit down in a meeting room for the detailed questions.
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.