I would put this kind of defect in the same category as off-center inlays and poor color registration in printing. If it's a subtle defect, probably not much value even to an error collector. Then again, if half the inlay were shifted or smeared, that's something that should NEVER get past the inspector, and probably would be worthy of including in an error collection.

What constitutes an error vs. poor quality is always going to be subjective. I kind of like to apply the "quantum difference" rule:

If it the kind of defect that is either there or not, than call it an ERROR.

If it is just a result of a process variation, and there can be many degrees of variation, then it's a DEFECT.