Smashed this bug myself - this might be a forgotten piece of code since it wasn't expected to affect anything...
There are two calls to show_replies() in showthreaded.php
One call has this guard:
if (isset($tree[$PNumber]['children'])) {
The other has no guard, so I added the following:
if (isset($tree[$current]['children'])) {
and it seems to be working correctly so far.
