During the height of witch-hunting in the 15th century, there was a book published in Germany that was a sort of "Witch-Hunting for Dummies" guide, name of Malleus Maleficarum.
Full page scans available at Cornell's online repository, and while you're there, they have a few other witch-related tomes to check out for all your witchery needs. There's also a full site devoted to this and other witch-related beliefs, even into the present day.
But even practices for catching plain old criminals wasn't much better. For instance, there was the process of cruentation, in which an accused murderer was brought together with the corpse of the presumed victim and ordered to lay their hands upon the carcass. If the dead body should then spontaneously begin bleeding from its wounds, that would be a sign from on high that the defendant was guilty. One can only imagine how many murderers got off scott-free.
Many such practices are covered in the blanket category of "Trial by Ordeal," where you get all the variations on tying you up and throwing you into the river to see if you sink or swim, or plunging your hand into boiling water to see if God healed you, or simply swallowing poison, or other such life-jeopardizing trials. In some cases, surviving the ordeal unscathed meant that God had declared you innocent, while in other areas it was considered just the opposite proof, that you had escaped by the Devil's aide.
Then there was the practice of compurgation, a law which meant that you could be found innocent if you could find twelve people who said they believed your side of the story. Well, who couldn't scare up twelve friends?
One more curiosity is the German principle of "stadtluft macht frei," a kind of statue-of-limitations where if a serf had managed to escape capture for a year-and-a-day, they was no longer open to being re-chained.
And for a final medieval law oddity, animals could be tried in a court of law exactly as if they were human.