I have, for years now, always pronounced the word cache as kaysh. I have encountered a few people over the years who pronounced it more like cash or even cashay and worse (catch?), and more often than not, I have brought them into the fold—so to speak—by convincing them that kaysh was correct. However, after something close to an argument with a grammar-Nazi (yes, you know it's true!) colleague, we decided to set the matter at rest by doing some research. Desafortunadamente, (and to my keenly felt embarrassment) I lost :/
After much digging around in numerous dictionaries, our (unanimous) conclusion was that the correct pronunciation of the word cache is cash!
It's been hell trying to continually correct myself :(
P.S. As a commenter has pointed out, it is possible that cache is pronounced kaysh in some parts of Australia (which, however, does not really explain how it's sneaked into my vocabulary). I have anecdotal evidence that this might be true.