Anchor link fires despite "return false"

I wasted a long time wondering why I couldn't stop the link for an anchor tag firing after the onlick Javacsript had run. Was it because I was setting the onclick function using window.onload? 

No - it was because I had errors in the function that I was calling. And because of that, it wasn't getting the the return false at the end (which is what stops the src link being followed by browsers with Javascript working).