It Seemed Trivial

You never tell me anything, so how the hell should I know what might be important?

—Michael, SSDD

This is when someone knows a fact that would be of great importance to the heroes but never says anything, for the simple reason that that person has no idea the fact is important. A subtrope of Poor Communication Kills.

Compare and contrast You Didn't Ask, which is where the person who knows the clue also knows its importance, but simply wasn't asked to divulge.

The revelation of the fact may bring about Ignored Expositor, with "It seemed trivial" (or some variant thereof) being the answer.

Examples of It Seemed Trivial include:

Literature

Discworld: Vimes has a moment or two of this in every book he stars in, but Feet of Clay and Thud! probably have the most.
In the Dante Valentine series, Danny is forever begging Japhrimel to tell her things. He doesn't until it comes up in battle. When Danny asks why he didn't tell her sooner, he shrugs it off answering, "I did not want to trouble you with something so trivial." He abuses this excuse to the point Dante is often ready to scream.
The crucial clue that Hercule Poirot needs to solve the mystery is usually something like this. It's lampshaded in The ABC Murders, when he summons the families of all of the victims precisely for the purpose of talking about the trivial things surrounding the murders, in hopes that there will be some small detail that connects all of them. There is, but it turns out to be a Red Herring.
Elminster in Myth Drannor had a scene with Elminster wandering the city for hours (and collecting a few invisible watchers in process) as he tried to find where a dying elf asked him to carry his heirloom. He got the memory-gem of a noble House, and as such could remember their mansion in details, since it was a part of memorable events... but of course none of the previous wearers bothered to add a map or instructions for something as trivial as finding their own home.

Live-Action TV

It seems like in House every other episode or so features this—the key turns out to be something that the patient would have gladly divulged (especially as an alternative to the humiliating but irrelevant parts of his life that they unearth while looking for the clue), but couldn't see that it would possibly be relevant.
The Columbo episode called "Lady In Waiting" has a wealthy woman kill her brother and make it look like she accidentally mistook him for a burglar after he tripped the alarm system. Her fiancé came to the house unexpectedly as her setup was unfolding. Only later did he realize (with Columbo's help) that he heard the three fatal shots first, then heard the alarm sound.
In the season one episode of Farscape, "I, E.T.", the crew is desperately searching for an anesthesia to use on Moya located somewhere on an alien planet, only to find out, at the eleventh hour, from a befriended local that it's a common spice they used in their food.

Video Games

In Tales of Vesperia, Judith knew the truth about the blastia and how they were affecting the planet. Did she share any of this information? No. And when Rita and the others asks, "why didn't you say anything?" Judith's response (as seen during one of the private scenes between her and Rita) amounts to, "it didn't seem important at the time."
This trope is pulled in spades in the Ace Attorney series, infuriating the player character to no end. Witnesses will sometimes not bother to tell the player a vital piece of information or give some important evidence that could help clear their name or solve the case because they feel the item/information has no bearings on the current situation.
- A particularly notable example is when Miles Edgeworth completely loses his composure in the middle of court over a playing card that Adrian Andrews has been absentmindedly toying with. She is genuinely confused when he demands to know where she got it and why she didn't tell anyone about it. Turns out it's the calling card of an infamous assassin, which dramatically changes the case.
In Tales of the Abyss, Jade deduced almost immediately that the "Luke" he was traveling with was actually a replica of the real Luke. Jade didn't tell because doing so would have resulted in massive amounts of existential angst on replica!Luke's part with no apparent beneficial trade-off. In hindsight, that wasn't the best decision. Revealing the truth would have stopped the Big Bad's manipulations, even if Jade wasn't aware of them.
- Luke does this later on in the last third of the game when he neglects to tell the rest of the party the local Deity had contacted him, warning him that the Big Bad had captured it. This one can be chalked up to simple ignorance, as Luke had no idea what the message meant.

Web Comics

In SSDD, Michael knows that British Intelligence created something called the Echelon Plug-in; he was given a copy of it. Unfortunately, Michael is generally treated like a Butt Monkey and has been so locked out of the loop that he doesn't know what the Oracle is or that it even exists. Of the group trying to prevent the Oracle from gaining control, Michael is the only person who is not from the future, but nobody else listens to him, so they don't know about the existence of the Echelon Plug-in. When they finally tell Michael what the Oracle is, Michael makes a comparison of the two programs and the others realize that the Oracle and Echelon Plug-in are one and the same. In other words, the Oracle already exists in Michael's time, which explains a lot.
Antimony Carver, from Gunnerkrigg Court. At the point of Chapter 33, she had enough information about Jeanne, one of the Court's founders, whose information was deliberately hidden or destroyed. This is even lampshaded at some point in Chapter 23 by Jones:

"A lot of information about the origin of the Court has been lost or, in my opinion, deliberately hidden. If you have somehow come across information from that time, it could provide valuable insight. (...) However, such information is useless until properly investigated. When you feel you have uncovered the whole story, please, tell me about it."

Real Life

Many a school shooting could be averted if not for this. Often times there's tons of evidence that person is going to do this but its knowledge is scattered in bits and pieces throughout the community. This why sociologists suggest residences of communities keep lines of communication open even if risking embarrassment or being seen as a gossip spreader.