What are some examples of teachers, whom you considered bad when they were teaching you, but whom you in hindsight consider great teachers?

I had couple of teachers who pushed us to the limit, strethed our  performance to the limits, exerted pressure on us, and gave us some tough times ... to the points that they were considered "bad teachers" 

One of them was my English language teacher in9th and10th grades.  I was only14 at that time, and this teacher (thanks to him anyway) went to the extreme of asking us to buy the TIMES magazine and to summarize the over article on middle-east conflict ... This took me3 days work to get the job done and I earned an A+ for that work. 

This teacher and many others deserve the credit for what Iam now ... and I expect my Englisj writing style is a clear manifest of what I am talking about !!