If you upload the fla, I'll have a play with it for you.
Here's the fixed file:
What I did:
1. opened the piano button
2. Deleted the thumbnail movieclip out of it
3. Dragged the bitmap of the piano thumbnail (not the movieclip) from the library to inside the button where I'd just deleted the movieclip.
So basically, I just replaced the piano thumbnail movieclip within your button with the piano thumbnail bitmap.
Not sure why you were getting the error - I'm sure I've put movieclips in buttons before.
Maybe someone else can answer why, but at least you can continue with your project now.
The simplest fix: Extend the frame of PianoThumb layer to frame 2. That should fix your problem.
My guess is that Flash thinks the object has been removed when you go to frame 2 as the button is not on frame 2 (probably garbage collected). When you return to frame 1, the reference to the object is already gone hence the error. Flash CS5 don't seem to have this error though.
Thank you Shaun * a million. @ teugene I tried that before I went to bed last night and was getting the same error. As I was trying to sleep I was being haunted by Flash errors, and I started to think about what I was doing wrong. I might need to import the thumbs as a flattened bitmap and not with editable layer styles. I'll try that too. Either way I can move forward now. I don't understand why a mc in the button would be a problem the second frame in my main fil is a button and the rollover image is an animated mc. Oh well. Thanks again.
Hey Shaun a quick update. I don't know why this worked, but it did. I imported the thumbs as a flattened bitmap and not with editable layer styles. Then made all the buttons and links, and it works perfectly. Your input helped tremendously.