First things first, please, no "Mac vs PC" debates. Ludum Dare might be the only gaming community where I haven't endured a "PC Master Race" flame war, it'd be nice if we could keep it that way.

So, I believe starting with the last LD, we saw a this issue a lot. I discovered the cause of it, a way for developers to prevent it from ever occurring, and a way for players to fix it after the fact.

The issue occurs either when a Mac .app is compressed into (or extracted from) a .zip file. This included the .zip that's automatically generated by itch.io.

So, the precaution that developers can take is to put the .app inside a .dmg Disk Image file before uploading. Even if the .dmg gets put inside a .zip, the .app will be protected.

For Mac users who've downloaded an already-corrupted .app, you'll need to do a bit of Terminal maintenance. Richt-click the .app, navigate to /Contents/MacOS, and find the single Unix executable file. Open a terminal window and type in "chmod +x ", then drag in the Unix executable. You should wind up something that looks like this:

chmod +x /Users/UserName/Downloads/GameName.app/Contents/MacOS/GameName

Hit enter, you should see the icon change, and your .app should be good to go!

NOTES: Check out the post below from @outfrost for a more accurate/detailed explanation of what's going on, and potential ways to get around it from Windows.