That feel when error messages lie to you.

[Azareel]

It appears when you try to exchange an item of the same type (ie, HP 100s with HP 100s), not the same shape.

Is this new, or have I just magically missed it all this time?

[Hatsunechan]

It's always been there.

[Azareel]

It's always been there.

Now I feel extremely observant.

[Kyubey]

I've seen a "you can't equip this item with heavy armor" a couple of times while switching gears, no idea why since i don't put heavy armor on for years already

[Iyasenu]

"Too far to gather."

/standing right on top of the herb/sheep/tree

[December]

To prevent scams like putting put a white Fixed dye amp and sneakily switching it with a regular dye amp which appears white

[Kyubey]

I've seen a "you can't equip this item with heavy armor" a couple of times while switching gears, no idea why since i don't put heavy armor on for years already

and found it, apparently if you drag any item to your falias slots that message appears

[GODZILLA]

"Too far to gather."

/standing right on top of the herb/sheep/tree

That is so very annoying.

[Azareel]

To prevent scams like putting put a white Fixed dye amp and sneakily switching it with a regular dye amp which appears white

Except it doesn't prevent that.
It only comes up for items of the same literal type. For instance, swapping a fixed dye with another fixed dye.