On a project I spent an hour trying to fix a bug with MovieClips not appearing. The cause was that I was adding one MovieClip first to one parent and then to another. Flash allows this, but since display objects can only have on parent, it silently removes the MovieClip from the first parent.
Not getting any error information, I didn't know what happened until it finally just "hit me".
In your opinion, who is at fault here? Me, for not taking enough care, or Flash, for silently allowing this instead of throwing an error?