I'm a TA and last year had to correct the teacher I was working with several times. I would never do it in front of the children, and luckily she was very good natured about it, and actually asked me to correct her if I spotted any mistakes in future. Some of her mistakes were pretty bad though. We were teaching year 3, so 7/8 year olds.
We read a book set in Africa once and the children had a list of questions to answer, one of which was 'Which country is the book set in?' The children all answered Africa and were all marked as correct! 
Another time, the teacher wrote a maths question on the board which began '5 boy's are going to the shop..'
She also misspelled giraffe as 'girrafe' and grizzly bear as 'gristly bear!'
There were many, many more, including lots of maths ones - by her own admission she was awful at mental arithmetic.
Thankfully, I have been working with a lovely new teacher this year who never makes mistakes and always double checks if she is unsure of something.