Never seen this menu before; Yes or No

[LightTrench]

When updating to 1.5 and installing the latest forge, I tried to load a world and was presented with this screen:

 

I have never seen this screen before. Clicking yes loads the world, clicking no crashes the game.

No other mods are installed and none were used on this world.

What exactly does this screen mean and why does clicking no crash the game?

[LexManos]

Im going to guess this is the id mismatch screen, typically it displays a list of mismapped ids.

You should be able to see more information in the console log.

[Noah_Beech]

Lol, yes...No? (clicks no, crashes)"WTF"

When you think about it, it is freaking hilarious

[LexManos]

Typically it is asking if you wish to continue loading the world, and that there may be possible corruption issues due to missing mods/items. Hence the no making your game exit.

As I said, check your log.