It becomes difficult to tell error vs artistic license. In some cases it's more apparent (the artist probably didn't mean to draw backwards feet) but things like eye or hair color might be intentional. It's not uncommon.
I think forgetting to make a character's eyes two different colors is on the same level as drawing someone's feet on the wrong legs. A very easy mistake to make, and exactly that, a mistake. I've never seen anyone claim to have drawn Tatara Kogasa with two red eyes or two blue eyes intentionally. The fact doing it right requires the slightest bit more attention than filling in the same color for both eyes, what you do the other 99% of the time, is what makes it so much more likely to be unintentional vs just giving someone entirely different colored eyes than they normally have.
Then what about alternate eye color (heterochromatic), to solve the issue of labeling it error or artistic license; the ambiguity around it being alternate; and to clarify that the character's eyes are not their usual color?
That name sounds like what happened is a character that normally doesn't have heterchromia has been drawn with two different eye colors.