<There’s a 22 year old thread on this, I thought better to make a new thread than ping all those people again>
In this thread, @solost gave the example of Columbo managing to incriminate the perp with a “swap the poison” style trick, where Columbo not being decapitated proved the perp’s intent to kill.
My example is going to be in the Jonathan Creek* episode Daemon’s Roost.
The killer is impersonating a security guard working on the grounds of a manor. The patriarch of said manor has had a stroke and cannot move his body, only his eyes. The patriarch is ultimately poisoned, and, as he’s dying, he sort of waggles his eyes.
Creek later figures out that he was actually darting his eyes between a phone and the letter Y, which, from his perspective, was the only letter he could see of a book’s spine.
Get it? He was spelling out the word “phoney” as a clue about the security guard! I mean, just staring at the guy would have been a better clue, but nah bro let’s shoot for the moon.
(I might have some minor details wrong, I can’t find a detailed synopsis)
* Jonathan Creek at its best was a series that managed to be both scary and irreverant, and with genuinely clever “impossible murders”. It dropped significantly in quality in later series though, of which this episode is an example.