Why is my application bundle sometimes missing its icon or crossed out?

Why is my application bundle sometimes missing its icon and/or have this "cancel" symbol over it?

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I have looked at the .plist file and the icon path and icon exist and are correct. If I right click the bundle and click Get Info, the icon shows up in the 'preview' section.

This happened after I moved around some code in my qmake file (but made no change semantically). Is this just a bug in Finder? There seems to be no way to refresh the view.

What generally causes Finder to display this cancel symbol and/or not load the bundle's icon?

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That's the "You can't run this application" icon.

The finder will display that icon if:

• the app is marked as being untrusted (i.e., downloaded like @kiamlaluno said)
• the app the wrong architecture (i.e., PPC app on an intel machine without rosetta)
• the application bundle is messed up (make an empty folder with an .app extension)

Since you're building this program, I suspect that the Finder noticed the .app bundle before it was finished compiling and thought it was broken. When you move your app to another folder, the Finder refreshed the bundle (which is complete now), and displayed the correct icon.