One of my peeves: THEN vs. THAN. It drives me crazy when someone writes something like, "I like Bill better then I like Steve," or "I'm going to the library than to the store." :facepalm: I see this more and more these days.
That's because, nowadays, the dialects in which "then" and "than" are pronounced the same are spreading. More and more people are growing up unable to hear any difference between the two words, because of the fact that "short a" is actually one of the longer vowel sounds in the English language, especially before n, and because "than" is a function word that tends to be unstressed (the "length" of vowels refers only to how they were pronounced in Old/Middle English, and has nothing to do with the timing and rhythm of modern speech). This has led to changes, over time, in how the word is pronounced and perceived.
THEN and THAN do NOT sound the same. (And neither of them sound like THIN.) THEN has a short e sound, like peg or hen or led. THAN has a short a sound like that, cat, plan or can.
Welcome to the wonderful world of phonology, sound shifts, and language change. The British, and some East-coasters, would insist the same thing about "merry", "Mary", and "marry", which are homophones to most of us Yankees.
The problem is that even just in the US there are enough different accents and dialects that picking "one pronunciation to rule them all", especially for vowels, is difficult. Pick one *educated* speaker each from California, Texas, Iowa, and Philadelphia, and you'll find four different opinions on what pronunciations are proper. Add in the British Isles, Australia, etc, and trying to find a unified pronunciation becomes hopeless.
Note that I'm only arguing here about how things are *pronounced*, not how they're written. For me, the pronunciations of "then" and "than" are identical, but in writing I keep them very strictly separate, and it is jarring enough to be a pet peeve when I see the two switched.