What is meant by the hidden deviation (al-laḥn al-khafī), and what is its ruling?

Among the subtle hidden errors (al-laḥn al-khafī) are those that do not change the meaning. Examples include:

Omitting or exceeding the proper length of madd (prolongation), such as in (الماء), (السماء), and (الضالين). Failing to apply ghunnah (nasalization) to the emphasized mīm and nūn, such as in (إنّما) and (الجنّة). Omitting idghām (assimilation), such as in (منْ مَّاء). Softening a tafkhīm (heavy) letter, such as softening the Qāf in (قَال). Repeating the letter Rāʾ (tarīr), such as in (الرحمن).