Bummed out

[Britney]

-adjective. Feeling really depressed and sad about an issue. gloomy; morose. Affected by unhappiness or grief; sorrowful; mournful. To be in such a saddened state such as when you miss someone or something that you cant stop thinking about that person or something

[PussyCat]

In New Zealand, bummed out can have a different meaning from just being depressed or sad.
It can also be commonly used here to describe someone who thought they were so right and very insistent about how correct they were about something, but then ends up being proved wrong.
When this happens you can say that person has been bummed out.
A similar meaning and often preceding saying to bummed out is the interjection, shame!. Indicating that the person should feel shame (or even shamed out) for being wrong when they thought they were so right.

[Justin Bieber]

When you are absolutely positive that you are right, but then end up being proved wrong.