A question mark in the dock normally means that the Dock can't find the program that it's referencing. This is normally caused when you run a program from a DMG file directly. What you *should* do with a program in a DMG file, is to drag that program to your Applications folder. Once it copies over, you close and eject the DMG. Then you run the program directly from Applications.
Yes. The ? icon on the dock means the application is missing. The most common cause of this is just as described above. However, trying to be helpful, if you click on a ? icon in the Dock, it looks in your Applications directory (where it should have been, but wasn't) and tries to find an application with the same name.
If it finds one, it launches it. But, it does NOT "re-pin" the new location (I guess being the idea that sometimes you might want to have an app on a removable disk and switch back and forth or something). In any case, this will "work" as described above:
I have a ? appearing in my dock which reverts to the MC Center icon when I click on the ?
You can use these ? icons to launch applications, if they are really installed correctly and just linked to the dock incorrectly.
While the application is running, it'll show the real icon. But as soon as you quit out of the application, it again is a "broken link" and so it goes back to the ? icon.
I see users do this all the time with DMG installers. To "install" the app, they might drag it to the Applications folder correctly (following the handy arrow-pointer thing the app developer included in the DMG's background image). But then they
launch it from the DMG immediately afterwards (or drag the DMG's icon to the dock, another common mistake). So, the app is really installed right, but the pointer icon on the dock is broken as soon as you unmount the DMG.