While looking at it here on the forum, there are differences, sure.

But in the heat of battle, when you only have <0.5s to glance down to see where the ability is and/or what time is left until it is usable again, it is very easy to mistake.

And yes, now that you point it out, Cure III does indeed look very similar as well! Now, obviously, you're not going to be looking at your cooldown timer left on something that has no cooldown, and that is why I never have a problem with Cure III, that and it (along with all of my other non-cooldown stuff) is on the main no-modifier bar.

The problem arises when two abilities that are cooldowns have somewhat similar icons, sometimes I glance down to check the cooldown (and/or sometimes click the button instead of using the Ctrl+ or Alt+ shortcut because I'm moving at the same time and don't want to/can't stop moving to hit Ctrl+ or Alt+), that I'll mistake one ability for another if the icons are too similar.

I haven't run into this on PLD because no two PLD cooldowns look similar.

And c'mon, SE. Could we move past the "silhouette of a person with glowy stuff around them" for icons? Surely you can think of something new for icons.