For a demonstration, I will say that I am viewing topic 1 in forum 2.

Pagination for forums works fine, but pagination for topics does not.

If I view the pagination for this topic from viewforum.php?f=2, I see these URLs:

./viewtopic.php?f=2&t=1&start=0
./viewtopic.php?f=2&t=1&start=40
./viewtopic.php?f=2&t=1&start=80
./viewtopic.php?f=2&t=1&start=120
./viewtopic.php?f=2&t=1&start=160

If I am within the topic and I view its pagination, I see these URLs(which point to the same pages):

./viewtopic.php?f=2&t=1&st=0&sk=t&sd=a
./viewtopic.php?f=2&t=1&st=0&sk=t&sd=a&start=40
./viewtopic.php?f=2&t=1&st=0&sk=t&sd=a&start=80
./viewtopic.php?f=2&t=1&st=0&sk=t&sd=a&start=120
./viewtopic.php?f=2&t=1&st=0&sk=t&sd=a&start=160

Search engines view these URLs as pointing to different pages, and it wastes search engines' time (because it makes them index the same content) and makes phpbb boards more difficult to search.

It would make more sense if the t, st, sk and sd variables were included in the pagination links for the topics displayed from viewforum.php.