I think it is possible that some speakers, with a mental picture of 'combination of knowledge and experience' might use 'is', and I also feel that many listeners would not notice anything unusual.
It seems unacceptable to me in writing.
Thank you for that. I knew logically it had to be 'are' and many arguments here took place about whether you could group the 2 things (knowledge and experience) together to consider it as 1 thing, and use 'is'. I'll have to let my colleagues know that it is 'are'.