If interviewer asked you, "What's the worst thing you've ever done?" would you know the answer right away?

YES,,, CHOOSING TO BE AN EMPLOYEE FROM THE BEGINNING OF MY PROFESSIONAL LIFE,,,,  IT MAY NOT BE APPROPRIATE ANSWER IN AN INTERVIEW FOR A JOB,, BUT FOR ME,,, THIS IS THE WORST ONE,,,,,,,,,,,,,,,,,,, GOOD LUCK

This answer will be very difficult for everyone because in this way nobody assesses his or her or someone hides the negative things from other. However, I was always absent last10 days before starting examination in my school, college and university. It helps me to prepare the exam.

"The worst thing I've ever done is that I didn't value time when I was younger and I didn't use it more wisely, so that I can explore all my possibilities. If I had done that, I bet that I could have studied something more significant cause I already had good grades".

One answer to one question like that does not fully define who I am, certainly not to a recruiter, unless it defines how authentic I am. I will share the first answer that comes to my mind and one that I actually CAN share, which is: 

The worst thing I ever did was thinking that neglecting social life for the sake of entrepreneurship is only a common sacrifice for every entrepreneur. I came to realize it is more than that. It is the letting down of those who truly care about you for what will always be seen as a selfish pursuit (even when it is not) if you do not explain it clearly enough. Support from a qualified comfort zone is essential. 

In my childhood, I hunted birds, even I never hunt a single bird, but one day I saw a bird on our house wall when I start to hit the bird with slingshot the stone hit my mother's eye but thanks Allah she was not hit with big force. After that my mother caught me and hit me with a hand I cried a lot and after that I stopped bird hunting.

This question describes your personality, for example,  if you have done worst in any task what changes you brought in to minimise the failures if the same project you will be asked to do again... The concept of this question is based upon risk assesment, are you a good risk analyser? how you see the uncertainties? your approach to tackle the comlex issue? Therefore, the answer to your question is a must "Yes", you should know the answer right away and the alternative methods to overcome the same situation if it happened in future again. 

You should always remaim calm when asked such question. That's one of those tricky interview questions. You don't want to say that you never did anything wrong, because nobody is perfect. On the other hand, you definitely don't want your "worst thing" to be something really bad, illegal, unethical, cheating, nasty, etc. You don't have to be all honest, you can use something you did when you were small.

For me " Yes ", Because many people have different stories to be told and to be shared, If that question is asked by me?. i can think lots of things that worst happen in my life. :) 

First thing that popped in my mind was "nothing, but I will have one if I don't get this job" -- lol but seriously, I can't think of anything. Probably because of how we interpret our actions as either bad or good. I think the worst that I can think of is to lend an invisible inked pen (ink disappears after some time) to a classmate during our finals exam.

Worst thing is working like a bull between sheep. They were not ready to become bulls and i was not ready to turn into a sheep.

Such a question is more a privacy issue, therefore, I won't answer it, but I will comment by saying if you mean related to work, I will say: once I had a good paid job but It didn't satisfy my needs and I felt I had nod added value to the job, so I resigned.