Spell Interuption.

That's what most people do. Even small amounts of lag will mess with stuff and make it think you are still moving after you haven't. If I'm in pt I try not to move as much as possible. Which means staying in casting range and out of AOE if possible if not meh. Though if your really good or got good tools you can cast while running if you run back to the exact same spot at the end lol

Don't stand on slopes either.

Also gear changing allows your character to move when it normally wouldn't. So like if you are on something you can move without trying. If you are bound you can move. Even if it's just a little

To clarify on the slopes thing, there's something in the FFXI code that causes your character to try to stay level with a surface. If you're on a sloped surface (Or, say, stairs, for instance), you will get a kind of poppy animation every 30 seconds or so, that's the game trying to argue with the code that the character is on a slope.

Basically, if you've ever fought a mob on a slope or had the death animation on a slope, you'll see your character stand at a 90 degree angle from the ground, which is what the game is trying to prevent, because it looks silly.

So you don't ever stand perfectly still on sloped surfaces, because everytime you cast, your body position moves.

I've never seen just standing on a slope actually mess up my casting though.

I mean theres plenty of strat out there on some mobs that are dependent on the backliners standing on slopes lol. Or not even strats but just no where else to stand and not be in AOE range

There's a specific degree of slope that causes it a lot. Seems to be around the 30-40 degree slope range. I get it a lot on the hill in the Northern Caedarva Mire camp, and also on stairs in Whitegate.

I haven't camped mire in forever or really casting on stairs that I can think of... bah must go and try it

Attowah Chasm is the best example, I'm sure we've all had it happen there. It's more common for casting to be interrupted by a slope when you blink.

Which side? I don't really go on east side (have a dislike for unecessary aggro and NMs that can beat the ***out of me) cept the part at the end to cut in but I go on the west side a bit like um everyweek and used to camp pets there and was fine.

And yeah everytime you blink your character tends to move a tad. More if it can. It's fun times when your bound and got no erase trying to get places blinking lol

I can think of the slope at Tiamat, my character moves about once every ten seconds there. Just takes the tiniest step forward visibly, never had it interrupt casting but it's a good example of moving on a slope.

Had cures interrupted once or twice by blinking on the slopes at Alastor Antlion's ???.

I have general distaste for that zone especially that side.

Side note though I haven't ptd in mire forever I did kite the T2 ZNM imp all around there for a good 1.5 hours np. Didn't dent it much with its super fast cast and longer casting range not with all the undead around anyways.

This is it pretty much. I have this problem occasionally with spells and with ranged attacks. Only suggestion is to stop, wait a half a sec, then do what you need to do. I usually only have this problem when I'm solo. Otherwise I have me arse firmly planted in one spot most of the time.
I think it has to do with lag. I've been knocked back mid-spell and it has still gone off before.
My character jumps around a lot when I talk to some NPCs near hills. Like the tags NPC inside Fort K-N_S.

From what I've seen it's more an issue of standing on the side of ledges rather than slopes. The clarification being you can walk up a slope, but you can only fall off a ledge. I was half-way down a ledge in dynamis-bubu fighting something, hit my disseverement macro, gear swap made me fall down the ledge and interrupt my casting.

As for your character moving on it's own (when it's not on a ledge), that's your character trying to fall down a crack or tiny ledge. Attowha Chasm is full of this sort of shitty terrain. If you walk up stairs slowly you'll notice you actually plant your foot on every step, but walking DOWN the stairs you'll hover above them, that's because each step is a tiny ledge that you're falling down (but small enough that you can step back up, which contradicts what I said earlier a little).

There are a few small ledges in Caedarva Mire that could cause your character to slip on a gear change, so as everyone else has said, make sure you fully stop for at least half a second, and don't stand right on a ledge before you cast and you shouldn't have any issues.

Oh, and Composure shouldn't be full timed unless you're meleeing something solo. In a party situation, turn it on, cast refresh > haste > stoneskin > blink > phalanx (or just the first two depending on the party) and switch it off. Recast is 5 minutes and those buffs will last 7 and a half. If you're meleeing solo, you can full time it for the acc bonus for pretty much everything aside from serious /nin tanking, if you're relying on those utsu recasts, composure is bad juju. Gets a little tricky to re-apply refresh/haste/phalanx/spikes/enspell when you're doing this (unless you can bind/grav/kite the mob) in a longer fight, but it's a damn site better than not using it. Thankfully I levelled my rdm well after composure was implemented :p