Lahn khafi occurs when the underlying letter can still be identified but an expected Tajweed quality is not correctly observed. It is called subtle because the error may not be evident to an untrained listener.
For example, the letter ر has contextual rules governing a heavier or lighter pronunciation. If the consonant remains ra but its prescribed quality is mishandled, the problem concerns recitational manner rather than substitution of a different letter.
Detecting subtle errors requires attentive listening, comparison with a skilled reciter, and gradual study of the phonetic properties of letters.