If you go for example here:
you will notice that "General" heading is hyperlinked to
Similar behavior exists in viewtopic, heading of the topic gets start=0 added to its url.
"start=0" is unnecessary and results in two pages being indexed by search engines for every actual forum or topic on a forum.
I found out about this through google webmaster tools. In html suggestions I was told that I have pages with duplicate titles. Sure enough, since the pages are identical their titles are as well.
Attached patch checks whether $start is equal to zero, and if so, does not append start=$start to viewforum/viewtopic urls.
Technically several other pages have this behavior too, e.g. viewonline and mcp. But since they are unlikely to be usefully indexed I did not bother changing them. On my site I have robots.txt prohibiting viewonline and mcp access, among other things.