Chronic Evidence Retention Syndrome

Ladies and gentlemen, I'd like to raise your awareness of a tragic disorder affecting everyone from common street thugs to criminal masterminds everywhere, Chronic Evidence Retention Syndrome. Responsible for the downfall of many otherwise competent bad guys, CERS can be recognized by some of the following symptoms:

The pointless hoarding of evidence against oneself even when there's absolutely no need to do so;
Keeping easily destroyed or otherwise easily discarded evidence intact;
Having said evidence in a location trivially linked to you; and
In extreme cases, actually going out of the way to make sure it's clear that something is evidence, and linking it to you.

Please note that retaining evidence of a crime where the evidence consists of, say, something valuable you wanted to steal, isn't symptomatic. In a genuine case of CERS, the evidence will have absolutely no value to anyone other than proving the holder guilty. So be on alert for these symptoms, and so long as a boss isn't the Shoot the Messenger type, let him know he may be making a critical mistake.

Generally a subtrope of someone simultaneously holding the Villain Ball and the Idiot Ball. Typically, one can expect the person to be a Smug Snake, thus making his eventual downfall all the more satisfying. If an underling does it, expect the Man Behind the Man to make sure he doesn't get a chance to do it twice.

Examples of Chronic Evidence Retention Syndrome include:

Film

In both comic and movie versions of Watchmen, Veidt for some reason didn't erase the computer files that detailed his plan to frame Dr. Manhattan for causing cancer and leaving Earth, and information on his ultimate plan, even though the Manhattan plan had been successful (so far as he knew) and he'd already kicked off the second, so there was no need to keep the files at all.
- It's possible that he wanted the files to be found so that his friends would confront him and thus leave New York at the time of its destruction. I mean, why else would the "smartest man in the world" keep them in a computer with not only an obvious password but a prompt that basically says "Password incorrect: you need to ADD ANOTHER WORD"?
Minority Report subverts and lampshades this. When John Anderton searches Leo Crow's apartment, he finds the bed is covered in photos implying that Crow kidnapped and molested dozens of children (including Anderton's own son). Then it turns out Crow is innocent, and the photos had been faked and set up solely to give Anderton motivation to kill Crow. Later, Detective Witwer examines the apartment, and immediately (correctly) deduces that Crow had been set up, on the logic that real criminals never leave behind this sort of "orgy of evidence".

Literature

In You Only Live Twice, SPECTRE not only kept a photograph that they killed a couple for accidentally taking, but they helpfully annotated the back that they killed the couple for taking it, thus providing a clue for James Bond as to where to look.
Note to the Reigate Squires in the eponymous Sherlock Holmes story: when the international intrigue hero who's been all over the papers is sacked out in your parlor, that's probably your last chance to destroy anything incriminating. Also, if said incriminating paper has a piece torn off, you might want to wonder about that.

Live-Action TV

In an episode of Hawaii Five-O, once the bad guys steal the evidence that Steve McGarrett's father had been investigating, they keep it for some reason instead of just burning or shredding it, which allows an employee to steal some of it and leak the info back to McGarrett, leading him to the person who ordered his father's death.
A heroic subversion in Burn Notice: after Michael Westen is framed for murdering his CIA liaison, the first thing he does is get rid of the murder weapon. By destroying it with thermite.
An episode of The Persuaders involved a crucial piece of evidence that the culprit couldn't work the will to destroy, despite his henchman's urging: A gift by Adolf Hitler himself for helping the defeat and surrender of France.

Video Games

Played with Phoenix Wright: Ace Attorney's bonus case, "Rise from the Ashes." The culprit keeps a critical piece of evidence hidden away which he's used to blackmail someone into doing his bidding; revealing this evidence when prompted will have dire consequences, but concealing it (temporarily) will force the culprit to tip his hand, and the new context in which the evidence is ultimately presented points the guilty finger at him instead.
Noticeable in Fallout 3 and Fallout: New Vegas. Whenever you get a quest to investigate someone, you usually just need to pick the lock of his vault or hack into his computer to find a note explaining his wrongdoing. It's also not rare to find someone's password written in a fairly conspicuous place.

Western Animation

Occasionally happens to some Scooby-Doo villains.

Real Life

Real Life: Richard Nixon and the Oval Office recordings.
More Real Life: A surprising number of Internet criminals (and office workers) either don't know to clear their browser history file and cache, or simply neglect to do so.