It is defined as a mistake and deviation from correctness when reading.
Its divisions: It is divided into two divisions which are:
1. Obvious and clear Mistake (لحن جلی ظاهر)
It is a mistake that occurs in the pronunciation that affects the accuracy of the reading, even if the Mistake occurs in the meaning or not. This type of Mistake occurs in the make-up of the word or in the letters, i.e., the reader pronounces (التاء) (الطاء). The Mistake can also occur in the vowels where one is read as completely different. etc. Changing a dhammah to a fat-h or kasrah or changing the fat-ha to kasrah or a kasrah to fat-ha. It can also be that the Mistake occurs with the sukoon being read like one of the three vowels. This category is named obvious and clear because it is easy for readers and scholars of the language to know the mistakes and point them out.
2. Obscured and Hidden Mistakes (لحن خفي مستتر)
It is a mistake in the pronunciation that pierces the reading, heard by an accomplished reader, and is not a mistake in the meaning, language, or grammar. Only those who know the Qur’an reading recognize these mistakes.
This type of Mistake has two divisions (ينقسم هذا النوع إلى قسمين):
The first: Mistakes are only known by those knowledgeable in Qur’an reading, such as leaving out an idgham.
The second: Mistakes are only known by skilled readers of the Qur’an, such as unwanted repetition of the (ر). Included in this category is the Mistake of reading the dhammah as between a dhammah and fat-hah, in that the two lips are not circled, nor are they protruded forward as is required in a correct dhammah. This is especially prevalent in the following words:
عَليكُم – أنتُم
The kasrah also can be mistakenly read in between a kasrah and a fat-hah. This is especially prevalent in the following words:
We must try as much as possible to avoid making mistakes within these categories and never commit them on purpose.