Nothing to change. A couple of very small errors at most, or errors only between thematically similar or easily confused classes.
1 · Minor error
Mainly correct, with some small erroneous groups of pixels over a small part of the cell, between easily confused classes (e.g. shrub / sparse).
2 · Reasonable error
Correction advised. Errors over reasonable parts of the cell between easily confused classes, or small groups of errors between classes that should not be confused (e.g. cropland / urban).
3 · Major error
Correction necessary. Any artefact (mosaicking, cloud shadows), an error over half the cell, or a reasonable share of error between classes that should not be confused.