Now, part of the problem is that LARP setting writers don't usually put as much thought into constructing a legal system that meshes with the realities of gameplay as they do to other things. This includes me, so don't think I'm going after any of the other games I've played or read about. Another part of the problem is that most setting writers aren't scholars of medieval jurisprudence or, for that matter, any other past or future era. So let's look at some general cases.
Quick terminology note: I don't know of standard terminology to differentiate things players resolve with their out-of-game ability from things players resolve with character stats. To keep this post remotely readable, "material" scenes or challenges use the player's ability to find clues, solve problems, or otherwise perform the task. "Representative" scenes or challenges are solved with things found on the character card. Situationally, I may prefer one or the other, but let's take it as read that I'm not making a value judgment or trashing that thing you like.
Investigating PC Crimes
Sometimes PCs break the law of the land - those laws that are so basic and necessary that they didn't require detailed design thought, like theft and murder. Some PCs are upright citizens, while others are knaves and blackguards of the worst sort - they're adventurers, what did you expect? Anyway, the odds are pretty good that a PC will run afoul of the law at some point, or if not, will at least be accused of same.
This opens the door to two issues: Investigation and Trial.
In the Marath Suvla criminal justice system, the people are represented by two separate but equally important groups: the Town Guard, who(Joking.) Anyway, investigation. Investigation is very much at odds with emergent gameplay (that is, gameplay driven largely by player actions, in which Plot reacts nimbly.) A good material investigation scene takes time and preparation. Tuning difficulty on a material investigation is art entirely without science; this is a universal truth of puzzle creation. Now imagine that the investigation, if successful, would incriminate a PC. How could the Plot-created interchange of player conflict possibly be fair in the eyes of both sides? Broadly, Plot's sole responsibility in PvP situations is to show no favor and make sure that the rules are obeyed both in letter and spirit. To phrase that another way: to let emergent play be the truth, no matter what.
propagateinvestigate crimes; and the Khedive, who might or might not do anything about it, depending on his mood. (Akathian jurisprudence is capricious.) These are their stories.
The realities of LARPing don't include bloodstains, fingerprinting or higher forensic analysis (remember, a single player might be one of several different characters), torn clothing, shell casings (or ballistic examination), damage to doors, furniture, or architecture... et cetera. I've had two experiences with a staff member attempting to establish clues that would have incriminated me or my team. Neither of these were positive experiences, but I'm going to describe them only in general terms.