Ditto to ”-able” vs. ”-ible”, ”-ance” vs. ”-ence”, “diarrhea”, “lieu”, “ridiculous”, and those French-derived “au” or “ua” words “restaurant”, “guarantee” and {the less-popular} “guaranty” (think “sounds like ‘carrageenan’, spelled like ’guar gum’”) and “bureau” (although I concede that the spelling of “bureaucratic” is admirably appropriate). Oh, yeah—and “appropriate”. (I want to spell it like “apropos”.)
I have to stop and think about “necessary”, (if there were two “c“s, it would be pronounced “neckessary”,) but then I get it.
I, too, feel like “cemetery” should end ”-ary” (it’s not pronounced “cemetury”), but curiously the American spelling of “gray” annoys me every time I see it; “grey” looks right.
I consistently mistype “their” (I guess I really took that ”‘ie’ except after ‘c’” rule to heart), “onomatopoeia” (think ”Poe”: ”. . . rapping, . . . tapping . . .”), “sequoia” (which should be easy, because it contains all five regular vowel graphemes exactly once), “facetious” (which should be even easier because they’re in alphabetical order), and “ophthalmology”.
I used to always write “therefor” when I meant “therefore”. Once I got a paper back with an “e” written in after each occurrence of the word and a note in the margin explaining the difference. I’m eternally grateful to that professor—I majored in mathematics, and rarely does one write a proof without at least one “therefore”. (I did worry for a while about all the consistently misspelled assignments I suddenly realized I’d turned in over the previous semesters.)
I’m glad you asked this question @NaturallyMe; it prompted me to come up with some new mnemonics. (Did I mention “mnemonic”?)