Case 1 — letter identity changed: ث is read as س. This is an obvious error because one consonant has replaced another.
Case 2 — vowel changed: fathah replaces dammah. This also affects the prescribed word form and is obvious error.
Case 3 — letter remains, quality changes: ra is pronounced heavy or light where the opposite is prescribed. This is the type of defect classified as subtle error.
Correct diagnosis directs the remedy: learn the contrast between consonants, or practise the specific phonetic rule governing the same consonant.