"It seems like an open-and-shut case... or is it?"
A line heard far too often in episode descriptions.
To put it simply, an "open-and-shut case" describes a case so devoid of ambiguity that solving it is an absolute given; the case is opened, then just as quickly, it's shut. The banality of such cases don't make for great entertainment, which is why it sees more action as part of an ironic One-Liner.
There's many variations of this ubiquitous Crime and Punishment trope, so we'll just give one:
We've got a defense lawyer with blonde highlights. She's a hot, young version of Perry Mason. One day, she gets a call from her friends in the Deep South. A black guy is only a few weeks away from the ultimate vaccination, which will stop him from getting any illnesses ever again by virtue of being dead. It seemed like (here's those dreaded words again) an "open-and-shut case":
Mr. Smith's wife and three children were found horribly murdered. Mr. Smith was arrested by the police, his shirt covered in his daughter's blood. He confessed, was tried and was sentenced to a permanent cure to his problems.
The lawyer investigates the case. She finds out that the corrupt sheriff was responsible, the confession was obtained under duress and the blood was from him cradling his dying daughter.
This one still gets going because forensic evidence, even today, is subject to interpretation.
