Alfred is on trial. He stands accused of murdering Frank. He was caught red handed with a knife that tested positive for Frank's blood. He was also seen dumping Frank's body into the river by Bob, Frank's brother, and Mike, Alfred's best friend.
It seems like an open and shut case - except for one thing.
Alfred lives in a world of MAGIC.
Alfred's lawyer tells him to play the 'Illusion Defense'.
The Illusion Defense plays thus - since Illusion Magic is rare but possible, any event can be called into question.
Alfred claims that when he picked up the knife, in his eyes he saw a bottle of water. He also claims Bob and/or Mike were tricked by an illusion as well, and that Frank's body could be anywhere by now. This is complicated by the fact Police Teams haven't found the body yet.
Assuming 1) In this world you are innocent until proven guilty and 2) that CCTV doesn't exist or can just as easily be tricked by illusion magic, how does the legal system protect against 'The Illusion Defense', or how does this Magic World determine the truth in criminal cases?
Edit - Regarding Magic
In this world Magic is used primarily amongst the Military and Elites. Basic fireball-esk magic is common enough, to the point where entire military divisions consist of entirely magic users. Less than 1 in 100,000 would ever manifest anything significant on the level of Illusions.
Elites also have access to magic artifacts which can focus an individuals Will and produce various effects. At lower levels it merely improves an individual's health. At higher levels, an individual can fly, lift buildings or in some cases conjure illusions.
There is of course also the omnipresent specter of Forbidden Magic which is where the greatest use of Illusion is likely to be. In short, the most common use of Illusion Magic will be by those committing or trying to cover committed crimes. The rest of the time there isn't really a need for it outside of stage productions.