There are some words where the addition or subtraction of the letter ’s’ makes no difference whatsoever to the meaning, or validity of spelling but makes you look like a right goof if you do it inconsistently. There are way more differences between AUS/UK and US English spelling than you would give credit for. And spell-checkers are bastards.
Not because they miss whole rafts of thing, though they do. And not because they seem to merrily flip languages at will, though that’s also a bit of a giggle for them, apparently. But because they will actively suggest you change something so that it’s incorrect. I mean, sure, they ask first. But do you know what that does? It introduces doubt. Doubt is not a good thing.* Especially if you haven’t slept in a while. And you’re staring at a small brightly lit screen in confusion, watching the words swim into one another and going through another breakup with the Oxford comma.
It always happens during your last run through. When you’ve proofread it backwards and forwards (literally), when friends, colleagues, even your dog — if you have a dog — has been persuaded to glance through it, when you’re just about to… whatever. It’s probably important. You’ll remember after you’ve had some sleep. You wrestle the spellchecker into submission by sneering at it and its attempts to trick you, only to realise all of a sudden you can’t for the life of you remember how to spell ‘cushion’.**
The important thing to remember at this moment is not to panic. Do not rush to the room your significant other is sleeping in and demand that they enlighten you. What you want to do is casually stroll into the other room and ask your housemate to spell it for you. They will look at you funny. That’s the price you pay. But they will start (slowly, already the doubt is beginning to spread), in a slightly questioning tone of voice, to spell: C, U… and the heavens (or the depths, or whatever is appropriate depending on your deity/nirvana/otherworldly entity of choice) will open and the light will shine upon you and all will be saved.
* You all know that thing where you’re staring at a word, or thinking a word and it suddenly loses all meaning? (There was an XKCD comic about it the other day. Okay, so it was about Skynet, but I think the understanding remains.)
It’s like the more you think about it, the less you’re capable of wrapping your mind around it in a meaningful manner. It’s the same with words you routinely misspell. It’s not that you’re incapable of learning the correct spelling. It’s that you become paranoid about it. You start to think, I always put ‘ie’ in weird so clearly it’s ‘ei’. Wait. I wrote ‘ei’ automatically, so that means it’s wrong and it should be ‘ie’. This is a circular path that brings no good, and for some reason despite the thoroughly fifty/fifty odds it seems to end with an unreasonable frequency on butter side down.
** Cushion is the worst word in the world to suddenly forget how to spell. Once you manage to forget that it starts ‘cu’ you are gone. You cannot look that shit up. You can’t find it in a dictionary. You can’t google it. Basically, you’re linguistically screwed. I highly recommend trying it some time.