Magic is a threat to any current legal system. The reasoning regarding these legal systems assume things like, say, the word of experts (experienced / proficient professionals who act as the voice on matter for the judges). Magical procedures should be accepted since otherwise they could taint the entire process leading to an evidence dismissal as being a fruit of poisonous tree (think the hard it is to make a court accept DNA proofs and hidden recording proofs).
For stuff like a murder, you need more than just an illusions test (the defendant claims to been under an illusion and picks a knife which he never used). You must check:
1. Was the user under the effects of Imperio? (This was a claimed defense in <PERSON> canon). If the suspected does not remember anything, consider also he being under effects of obliviate.
2. Was the matter/space correspondence altered so that the suspected actually grabbed a water bottle, and one split-second later he was found with a knife in his hand?
3. Was he posessed by a summoned spirit? Either by him or someone else.
And questions like that. For that to work, experts should develop metamagic skills (spells that may affect or guess stuff about other spells, in the same way that meta.SE sites discuss about the corresponding SE site; your magic world should support some stuff like this in order to have at least a draft of a legal system). And then we fall into a Petitio Principii: Does metamagic affect itself? Is there a last-word metamagic spell which could unveil the truth about the execution of a metamagic spell (or regular spell) and not be countered/altered anyhow?
Magic is the ability to change the reality somehow. 805
Even if you limit your magic to just violate thermodynamics, it's enough to develop science breaking laws of space and time (e.g. teleport) and would screw your proof system. Even with metamagic, you find yourself in a world where everyone is potentially self-sufficient in power (or at least those who can handle the power).
Think about bitcoins. They are a coin generated by the network itself, with a kind of algorithm (programming could be seen as magic if some laws are respected) belonging to a scheme called Byzantine Fault Tolerance. The same would occur to the magic: the most people uses metamagic to freeze or recover the actually occurred facts (at least 51% of magical skills in the whole world), the most accurate will be the rules. Bitcoin nodes/servers are the wizards here, which process the transactions and maintain the encrypted wallets.
But being that a judge should correlate evidences -by logical reasoning- with facts by stating time, space, and matter being inmutable... What's the relevance of a trial as we state now when those three aspects of reality itself can be mangled? E.g. how would you implement a magic-aware legal system in a country with conflicts like Venezuela? (currently 66%+ people opposes the government and the overall system).
Summary: If the legal system -our current, Roman-based, legal systems- is flawed as it is now many times, and if an evidence corruption can flaw an entire case... How would this system be relevant/accurate when the reality itself is/was/will be mangled? Usually facts... happened. How would you trust any legal system if you can even make (with a combination of magic and our current science status) facts unhappen? There would be no fixed reality to judge. 693