The body of Moses

But when the archangel Michael, contending with the devil, was disputing about the body of Moses, he did not presume to pronounce a blasphemous judgment, but said, “The Lord rebuke you” (Jude 9).

According to some church fathers, this is an allusion to a pseudepigraphal work: the Assumption of Moses or Testament of Moses. Since the book is no longer extant, we can't directly compare it to Jude's statement. 

However, this still raises questions about Jude's use of that suspect material. In principle there are three explanations:

1) There's a liberal explanation. Jude was gullible as well as careless about his sources. He believed this apocryphal story was true. I only mention this explanation for the sake of completeness, as well as to provide a point of contrast for two orthodox alternatives.

2) Jude isn't crediting the historicity of the anecdote. Rather, he's using it the way a contemporary Christian might use an anecdote from a popular science fiction movie or TV series, or a comic book superhero, to illustrate his point. 

That explanation would be consistent with the inerrancy of Jude. And since we can't interview Jude, we don't know for a fact how he viewed this material.

3) Jude was crediting the historicity of this incident, and he was correct. Let's consider a few points:

i) Although the immediate source may be a pseudepigraphal work, that, in turn, is glossing a canonical source. For the passage is paraphrasing Zech 3:1-2:

Then he showed me Joshua the high priest standing before the angel of the Lord, and Satan standing at his right hand to accuse him. 2 And the Lord said to Satan, “The Lord rebuke you, O Satan! 

As in Jude, you have a high-ranking angel rebuking Satan in the name of the Lord. 

Also, in Dan 10, you have the Archangel Michael contending with a territorial spirit. So, to that extent, Jude's source has a basis in OT history.

ii) It's not unlikely that Satan would lay claim to the body of Moses. His corpse would be a prime candidate for the veneration of relics. Corpses of holy men, even reputed corpses of holy men, become the basis of shrines, pilgrimages, and prayers. 

So the proper disposal of Moses' body is religiously significant. Making his corpse inaccessible–in an unmarked grave (Deut 34:6)–forestalls the development of a religious cult that rivals the true faith.