Is it very unprofessional to contact people from an old job interview and tell them that I think I've made a mistake? One year ago I withdrew from one job application. I met the committee, the interview lasted nearly 2 hours, they showed me the institution, I had lunch with one of the senior team member. Then I suddenly withdrew before results were announced. The job was in a different country and I haven't got an opportunity to meet anybody again. After some time I see that it was a mistake. 

Is it a good idea to send them an email that I regard my actions as a mistake or as unprofessional? Let say to the senior member with whom I had lunch, or to the head of the project. 

I have  a feeling that I have made a mistake and it was maybe too harsh to withdraw. Maybe in the future they will seek another sort of collaboration or there will be another job available at the institution.
Dropping out of a search isn't really unprofessional (unless you told them to go to hell I guess - but then it will be way too late to apologize). In that light, and under the assumption that you just dropped out suddenly and politely (as most people do), mailing them is not so much unprofessional as much as it is pointless. As Henning says in a comment: "What are you trying to achieve?"


Get a job with them? Way too late for this - the previous position is either filled or allocated differently. If a new position opens up, you can always apply again.
Apologize? No need for that - people drop out of a job search for all kinds of reasons all the time. They have almost certainly moved on, and so should you.
Just let them know that you now regret your decision? Why would they need to know this, or even care?
Start a collaboration? "Hey, I dropped out of your search a while ago" is hardly the best ice breaker.


We all make mistakes. It's best to learn to let things go.