I lifted this description of Cassandra Truth from tvtropes.org
Sometimes people just won't believe you.
You try your hardest to tell someone that your parents are actually super villains and that you need their help to bring them down, or that aliens have landed in your backyard and are now plundering your kitchen, or that the nice grandpa in the flat above you is in fact an evil bloodthirsty creature from another dimension, but the authorities look at you like you are crazy and send you packing. What's a lone protagonist to do?
A common staple of Disney and children's films, where the Kid Hero stumbles upon an evil conspiracy or a criminal ring and their parents and the police refuse to listen. The only thing to do is to save the day yourself, with PG heroics and Scooby Doo style.
This sort of situation can also be used to maintain the Masquerade: if a minor character finds out and tries to tell someone that the guy next door is secretly a Warrior of Justice, they won't be believed, because who would ever think that that foppish playboy could really be the dark, grim Super Hero? Often though, it's because they're horrible at wording it so it sounds genuinely insane.
Sometimes Character A actually asks what's going on and, when given the true-but-bizarre explanation by Character B, responds "Well, if you don't want to tell me, just say so".
Related to Devil In Plain Sight, except that in that case, the disbelief is mainly due to the deceptive abilities of the "devil", whereas in this one, it's usually due to strange circumstances, the perceived unreliability of the speaker, or just plain bad luck. It can also be a Crying Wolf situation, where the fact that the character lied previously is obfuscating the fact that they're telling the truth now.
The title comes from the mythical seer Cassandra, whose prophecies were always accurate but never believed due to a curse from the god Apollo, thus making this Older Than Dirt. (Writers of speculative fiction just looove to name precognitive or clairvoyant characters "Cassandra" or some variant thereof. Yeah, Smallville, Buffy, Red Dwarf, and X-Files — We're lookin' at you.)
If the person telling the truth is an NPC in a video game, they're spouting Infallible Babble. In this case, the in-game characters won't believe them, but a Genre Savvy player will.
If the character is the one guy to figure something out despite all the much more expert people working on the problem, he is an Einstein Sue.
This is often played to a ridicule the doubters, even when the truth is on the level of the absurd. A protagonist warns others of a completely unlikely, nigh impossible event, but the audience, in on the secret, perceives a truth they would normally doubt themselves as completely obvious.
Subtropes are The Jor El and The Cassandra, where the character in question is in a position where they really should be believed, due to authority or track record, but still isn't. See also Not So Imaginary Friend, for a specific situation where the "truth" is the existence of a character. When truth in question has something to do with mediumAwareness, compare Audience What Audience.
Contrast with Sarcastic Confession and You Wouldnt Believe Me If I Told You. A particularly cynical twist is when whoever wasn't listening concludes that Cassandra Did It when what she's saying comes true.
The Other Wiki even has an article about Cassandra Truth."
I often feel depressed when someone says the truth yet no one believes him/her. Eventually, this will cause me to not watch the movie/TV episode again...I just can't stand to see the character go through it again. Of course, when its played for laughs, like in Phineas and Ferb, then it's OK.
So, what are your thoughts?