Surah 54:1:
The Hour has come near, and the moon has split.
Verse 1 claims the moon was physically split, and verse 2 acknowledges the reaction of the witnesses: "And if they see a sign, they turn away and say, 'Continuous magic.'"
If the moon—a celestial body visible to half the globe at any given time—had been physically cleaved in two during the 7th century, it would have been a catastrophic astronomical event observed and recorded by multiple independent civilizations (such as the Byzantines, Persians, Chinese, or Mayans). Yet, there is zero contemporary historical or astronomical record outside of later Islamic Hadith traditions documenting such an event.
A polemicist can use this to challenge the empirical credibility of Islamic miracles. Contrast this with the miracles of Jesus (like the feeding of the 5,000 or His public crucifixion and resurrection), which occurred in specific historical settings, witnessed by thousands, and were documented by both sympathetic and hostile contemporary sources. You can argue that the moon splitting is a mythological claim that fails the objective historical scrutiny demanded by Surah 46:4 ("bring a remnant of knowledge").