This is a facinating question Bagi
I think I have had two: the first one was when I had to get my certification as a Systems Engineer that specialise in Internet, I did finally get the certifications, I think I had to pass about 8 or 9 exams, I failed one of the exams twice
The second challenge is still on going so I will not talk about it now, when I get there hopefully this year, I will come back to this tread and update it