How it's said/how it should be said-----smart alek comments
diptheria /diphtheria -----The ''ph'' in this word is pronounced [f], not [p].
expresso /espresso -----While I can't express my love for espresso enough, this word was borrowed from Italian well after the Latin prefix ex- had developed into es-.
excetera /et cetera -----Latin for "and" (et) "the rest" (cetera) are actually two words that probably should be written separately.
For all intensive purposes /For all intents and purposes ----The younger generation is mispronouncing this phrase so intensively that it has become popular both as a mispronunciation and misspelling.