One that I've always found puzzling is the two different ways of pronouncing "PRIMER" in American English..
There's the elementary textbook, and the special paint that you use to seal the wood so you can use the finish paint.
British people pronounce them both "primer", the first syllable pronounced long, just the same as "prime."
But Americans always pronounce the paint, that same way, but the book is pronounced "primmer" by some people and "primer" by some other people. And yet it is essentially the same word; i.e. something that is preparatory ??