DO NOT FRET HUMAN THIS PROBLEM HAS BEEN FIXED! NYEH

Uh, the sprite is red in the editor, see? You have to have to correct sprite set.

Okay, so open the editor, and go to the objects area. (It's the little green bug icon)
Search "flag". It's called "Goal Pole's flag" right now.

You should see the name of the object. To the right of the name you should see numbers: 16-1.
So memorize that. 16-1.

Check under the Save icon for a little palette tab. Click on that, and you should see under it:
Options, Graphics and Sprite Sets.

Click on Sprite Sets. Now, there's a few dropdown boxes with numbers to the left of them.
Click the dropdown box with the 16 number to the left of it (16-1, remember?) and then press "1: Flag, line platforms". 16-1.

That should solve it. This applies to every object. If any other objects go red, just place a pipe into an underground area and put the flag there.

Good explanation, though the extra area doesn't necessarily have to be underground or entered via a pipe. You can just put it as an extension of the main area - entered any way you like. Though Nintendo does usually use pipes

I know heh, but considering the format of his level, I'd suppose it'd be better for it to be a pipe and go underground.

Thanks.