...never trust a placeholder.
In graphic (and web) design it's fairly common to use a 'placeholder' image as a substitute for the image which the client or photographer still has to supply. With the placeholder in erm... place you can confirm that the overall design works.
In 'Keep On' I used a placeholder bass part in the demo recording featured here. The bass part was nothing more than pedal notes played on every beat - D D D D A A A A etc..
But of course, whenever I work on the drums, or the guitar or the voice, the placeholder bass is exploiting my lack of attention to inveigle itself into my affections.
Now, whenever I think of the song I can hear that plodding bass and my mind tells me that's just the way it has to be. Meanwhile, the placeholder bass is sitting in the corner, casually swinging a watch on the end of chain; back and forth, back and forth...