The. Elder Wand ownership
Watching again, and I just noticed...
As Voldemort understood, only the one that kills the previous owner may possess/use the Elder Wand. Since Snape killed Dumbledore, Voldy must kill Snape.
BUT - he doesn't kill Snape, really. He paralyses (?) Snape, but then order Nagini to actually kill him. Therefore, by Voldy's logic, Nagini should actually be the true master of the wand. Oops! Guess you'll have to kill your own horcrux now!