The Qur'an states that Allah is the greatest deceiver (Surah 3 verse 54). The question is if he is not always truthful (like the God of the Bible who cannot lie (Titus 1:2; Hebrews 6:18)), then can he be trusted at all? How would one know when he is actually lying or telling the truth? How could anyone believe him about anything if his character is deceitful? (more on this below) If he is not trustworthy, then neither is the Qur'an. It is no wonder then that the Qur'an contradicts itself.
“The Quran affirms the inspiration, preservation, and authority of the Christian scriptures (including the Torah and the Gospel). Yet the Quran contradicts the Christian scriptures on fundamental doctrines (e.g., Jesus' death, resurrection, and deity). By affirming scriptures that contradict Islam, the Quran self-destructs." - David Wood (see video below)