During my first-ever technical interview, the interviewer asked me "Do you know the FizzBuzz problem?"
"Uhh, not really." (I was just thinking ok this problem has a name, must be some algorithm problem)
"So the problem is basically to give you the numbers 1 to 100, if the number is divisible by 3, print 'Fizz', if divisible by 5, print 'Buzz', if divisible by 3 and 5, print 'FizzBuzz'. For other numbers just print out the number itself."

After hearing the problem, I felt so many ideas popping out of my stressed brain.

I thought for a bit and said "ok, so if the digit sum of a number is a multiple of 3, then the number is divisible by 3, and if the last digit is either 0 or 5, it's divisible by 5."
Then I started to code out my solution until the interviewer said "there's an easier solution. Can you think of it?"

This stressed me out even more.

I thought for a bit and said "well, starting from 3, keep a counter that records how many iterations are done after 3. When the counter hits 3, that number would be divisible by 3 for sure. Should I try this solution?"
The interviewer said "Sure." So I started again.

However, I struggled for about another 3min until I realized this solution is a lot harder to implement. The interviewer probably saw my struggle too.

This was the point where he stepped in and asked me "Ummmm there's an easy way of solving this. Have you heard of the MODULO OPERATOR?"

In sheer embarrassment, I finished the code in 30s.

Of course, there was no further question after this, and I felt the need to seriously reevaluate my intelligence afterwards.