Seems rather specific. As in, apparently it requires active non player characters, excluding myst-likes. But we'll roll.

Does this include stories where whether the delusion is a delusion is ambiguous? (E.g. Fran Bow)

so our game must adhere to the guidelines provided? I have ideas which fit the theme of "a delusion" but was hoping I could take it in a different direction than those you've suggested.

Thanks!

In the case of multiple main characters, do they all need to be under the delusion?

Must it be self-delusion or could it be deception, e. g. Total Recall or The Matrix? 

Do you accept full-screen lag?

I was going to participate in this jam, as I love adventure games and making adventure games, but I think the theme is a bit to specific. If the theme was just A Delusion it would be fine, but your description of that is more like an actual games interpretation of that theme. I wanted to make a first person game like Myst, since that's what I'm good at and I consider Myst the best adventure game of all time, since it practically started the genre, but now I don't know if I'll be able to do that since the only way to make your character seem to have a different idea to the player would be to have them speak. And I don't know about everyone else, but that is quite a bit of work for a jam and I don't just have some free voice actors readily available. Maybe next time the theme can be a bit less specific? The best themes for jams are when they are very broad, but still specific enough that you have to only start making the game when the theme is announced. A short phrase usually works well. Some examples of themes for jams I have participated in are "All's fair in love and war", "On the other side of the coin" and "Myths and Legends: Far away and lost". Those are very specific, but allow the game dev to interpret them in a lot of different ways and yet still be specific to the theme. Your theme seems a little restricting to the game that you can actually create, meaning a lot of the games may have similar ideas.

I like the theme, but the fact that the player must know that the character is wrong is... strange. I think it's difficult to have a player "fight" for the character if they feel like it's useless.

So, how blatantly false must the belief be? Must it be to the point that there's no chance of twist ending? Because I, as a player, would probably still keep the hope that in the end the protagonist turns out to be right.

I understand that the protagonist is wrong, that's fine. 

Can the protagonist know that he or she was wrong towards the end of the game?
Can he/she have that epiphany kinda like 'oh, I was wrong all this time and you guys were right'?

I think that would make for a nice redemption arc for the character, and the player would feel rewarded for 'helping out'. 

Very interesting theme. I now have an idea in mind. The main character is obviously wrong but he still insist that he is right until the end...thank you.