Is there any statistics available on forecasting failures? Like what percentage of financial/economical forecast fail? 
Or how often do politicians create bad policies based on wrong forecasts?
Like what percentage of financial/economical forecast fail? 


Well it depends on what you call a forecast.

Considering that there is a large uncountable number of people who are called (or call themselves) financial,economic and policy analysts all over the world who together make what one would think millions of forecasts made everyday, there is probably a number which fail and a number which succeed. 

So where are the numbers on it?

I would suspect no general statistic exists because its measurement does not have a practical use from a policy perspective. I say this in the sense, that it is a concern exclusively of private businesses and companies to care how well their forecasts work. providing statistics on their forecasting success would be exclusively for marketing purposes.


  Or how often do politicians create bad policies based on wrong forecasts?


This part of the question I recommend for Politics SE as it is  more political than economic.

Hope this helps