Ok here is something for Mysteries that I wish more writers/directors would follow
Do NOT introduce new information, unless it is inconsequential, in the final section. It is the laziest thing you can do to make a mystery tight and near unsolvable purely by hiding the information from the viewer (sure Scooby Doo did it... but most of those weren't REALLY mysteries).
Now this isn't because "The viewer must be able to solve the mystery", though that certainly aids in what I say, but rather because think of what you are doing from a storytelling perspective. The Case is unsolvable! The hero is finished and there is nothing he can do...
The villain is put into the same room as the hero, his head hanging in defeat... and then, instead of a nice last second trick or breakthrough... The Hero suddenly unloads a bunch of evidence from his butt that he apparently had the entire time and immediately solves the case... PHEW it sure was easy!
Now there are some shows that do AT LEAST give last second reveals but do you know what they do? Actually show you the evidence before telling you what it is. Heck Sherlock Holmes is often a reverse of this, they show you the evidence but only he can tell you what it means. They don't suddenly pull from thin air that the villain was wearing a blue lapel the entire time.
---
To put it in context I am watching a police procedural show (BOSS)...
In it a student in a jealous rage murders her teacher who is dating three people (One teacher, and two students). She calls her friend who agrees to cover it up.
So she does a genius job at hiding the evidence (Bleaches the blood, heats the body to give false TOD, covers up finger prints, and even plants evidence on a suspect). When the police arrives she immediately puts suspicion on herself and an unrelated person, all this to keep her friend out of trouble.
And... It works... Pretty much completely. So how do they catch the real killer?
Ohh right by revealing... She was the murderer the entire time! How did they find this out? By making up new information last second to implicate her (narratively speaking). This isn't even ignoring the sheer amount of new evidence they dump on you all at once (90% of the evidence is given last second).
It actually hurts the mystery to have the girl who went so far to save her friend end up being the murderer. As well to make the motivation for BOTH murders to be jealousy.
The episode is focused so much on making you hate the villain... that it forgot it had to have a mystery or drama in there. It also required the villain to pretty much be an idiot constantly.
You know... The Pink Panther had a bunch of evidence last second... but that was a joke (and the case was solvable)