Will a person be held accountable if he makes a mistake completing a Quranic verse he hears from the television, or if he utters words containing disbelief—Allah forbid—while trying to complete the verse?

The prescribed act is to listen attentively to the recitation. It is permissible for someone who has memorized the verse to complete it. However, for someone who cannot recite it properly as required, or does not know its continuation, they should either repeat what they hear from the reciter or remain silent. It is not permissible to attribute to Allah what one does not know. The Muslim Ummah is bound to correct the pronunciation of the Quran's words and to establish its letters according to the manner received from the imams of recitation. If someone makes an error in recitation and utters words that imply disbelief, but then corrects themselves and seeks forgiveness, disbelief does not attach to them.