At the beginning of 4th grade of primary (elementary) school, our teacher gave us a big spelling test. One of the last words was one I spelled conshienshus. He marked it wrong, of course. He gave us the same spelling test the next year (I had the same teacher two years in a row). Either he’d told us that it was going to be the same spelling test, or I’d decided that I’d swot up on that word just in case it was going to be in it. I got it right the second time. Even now, I often have to mentally say con-ski-en-ti-ous, in order to remember the spelling. The etymology is Latin con + scire – conscience comes with knowing.
sci- pronounced /ʃ/ is a rare occurrence in English. I can only find conscience, prescience (which I knew), nescience (which I didn’t, but which I could guess (WordPress’s spell-checker doesn’t recognise it)), conscious, luscious and their derivatives. All other -science words are actually about science. -ti pronounced /ʃ/ is a very common occurrence. Without doing too much research at this time of night (just after 11 pm), I think that all -tion and -tious are pronounced /ʃ/. (There’s a technical term for this.)
I was reminded about this during class, when one of the words on a list of adjectives of personality was conscientious.