It depends on what type of "mistake" you're talking about. Small mistakes (like not knowing some bit of crystallized knowledge, or getting a single answer wrong) shouldn't cause you to write off the candidate entirely -- everybody makes small mistakes, and some get performance anxiety worse than others.

So if the person messes up during the interview, if they seem smart enough, you can take the interview to the end, and possibly schedule a 2nd interview and see if they don't make similar mistakes that 2nd time.

Or you can decide against the candidate based on the small mistake, until you find your perfect, perfect candidate with the beautiful rainbows coming out of their resume,

the type of candidate that poops rainbows. Yes. Keep looking for him. And keep looking for him forever because you'll never ever find him.

But

If you find yourself interviewing a programmer who's not really a programmer (as in your example, "for a developer role ... discovered they can't explain a simple for loop") then in the interests of your own time (you're running a business aren't you?), you should find a way to conclude the interview early and move on to the next candidate.

If the candidate basically misrepresented themselves, then you owe it to yourself to cut the interview process early and go do some other work. If they don't have the skills they say they have on their resume, you're wasting valuable business dollars, that you should spend on interviewing other candidates instead. That doesn't mean you should be rude, or end the interview after the mistake abruptly. You can make a bit of small talk before closing off the interview, and be nice about it.

A way to end an interview early, would be to say something like:

"Thank you for your time, Johnston. We interview remaining candidates and get back to you if you have made it through to the next round."

It depends on what type of "mistake" you're talking about. Small mistakes (like not knowing some bit of crystallized knowledge, or getting a single answer wrong) shouldn't cause you to write off the candidate entirely -- everybody makes small mistakes, and some get performance anxiety worse than others.

So if the person messes up during the interview, if they seem smart enough, you can take the interview to the end, and possibly schedule a 2nd interview and see if they don't make similar mistakes that 2nd time.

Or you can decide against the candidate based on the small mistake, until you find your perfect candidate, who you'll never find.

But

If you find yourself interviewing a programmer who's not really a programmer (as in your example, "for a developer role ... discovered they can't explain a simple for loop") then in the interests of your own time (you're running a business aren't you?), you should find a way to conclude the interview early and move on to the next candidate.

If the candidate basically misrepresented themselves, then you owe it to yourself to cut the interview process early and go do some other work. If they don't have the skills they say they have on their resume, you're wasting valuable business dollars, that you should spend on interviewing other candidates instead. That doesn't mean you should be rude, or end the interview after the mistake abruptly. You can make a bit of small talk before closing off the interview, and be nice about it.

A way to end an interview early, would be to say something like:

"Thank you for your time, Johnston. We interview remaining candidates and get back to you if you have made it through to the next round."

It depends on what type of "mistake" you're talking about. Small fuckups (like not knowing some bit of crystallized knowledge, or getting a single answer wrong) shouldn't cause you to write off the candidate entirely -- everybody fucks up and some get performance anxiety worse than others. So if the person fucks up in the interview, if they seem smart enough, you can take the interview to the end and possibly schedule a 2nd interview and see if they don't fuck it up that time. Or you can decide against the candidate based on the small fuckup, until you find your perfect, perfect candidate with the beautiful rainbows coming out of their resume,

the type of candidate that shits rainbows. Yes. Keep looking for him. And keep looking for him forever because you'll never fucking find him.

But

If you find yourself interviewing a programmer who's not really a programmer (as in your example, "for a developer role ... discovered they can't explain a simple for loop") then in the interests of your own time (you're running a business aren't you?), you should find a way to conclude the interview early and move on to the next candidate.

If the candidate basically misrepresented themselves, then you owe it to yourself to cut the interview process early and go do some other work. If they don't have the skills they say they have on their resume, you're wasting valuable business dollars, that you should spend on interviewing other candidates instead. That doesn't mean you should be rude, or end the interview after the mistake abruptly. You can make a bit of small talk before closing off the interview, and be nice about it.

A way to end an interview early, would be to say something like:

"Thank you for your time, Johnston. We interview remaining candidates and get back to you if you have made it through to the next round."

Here's to candidates who can shit rainbows.

It depends on what type of "mistake" you're talking about. Small mistakes (like not knowing some bit of crystallized knowledge, or getting a single answer wrong) shouldn't cause you to write off the candidate entirely -- everybody makes small mistakes, and some get performance anxiety worse than others.

So if the person messes up during the interview, if they seem smart enough, you can take the interview to the end, and possibly schedule a 2nd interview and see if they don't make similar mistakes that 2nd time.

Or you can decide against the candidate based on the small mistake, until you find your perfect, perfect candidate with the beautiful rainbows coming out of their resume,

the type of candidate that poops rainbows. Yes. Keep looking for him. And keep looking for him forever because you'll never ever find him.

But

If you find yourself interviewing a programmer who's not really a programmer (as in your example, "for a developer role ... discovered they can't explain a simple for loop") then in the interests of your own time (you're running a business aren't you?), you should find a way to conclude the interview early and move on to the next candidate.

If the candidate basically misrepresented themselves, then you owe it to yourself to cut the interview process early and go do some other work. If they don't have the skills they say they have on their resume, you're wasting valuable business dollars, that you should spend on interviewing other candidates instead. That doesn't mean you should be rude, or end the interview after the mistake abruptly. You can make a bit of small talk before closing off the interview, and be nice about it.

A way to end an interview early, would be to say something like:

"Thank you for your time, Johnston. We interview remaining candidates and get back to you if you have made it through to the next round."