When calling gnus-summary-articles-in-thread with an article that is
different from (gnus-summary-article-number), it can return incorrent
results:

[post]
  [reply1]
    [reply2]
  [reply3]

When calling gnus-summary-articles-in-thread with post's id as argument,
while the point is at [reply2], gnus-summary-go-to-next-thread will jump
to reply3 causing gnus-summary-articles-in-thread to stop searching for
messages right before [reply3].
I think this behaviour is a bug, since it doesn't conform to the
function's documentation.

I attached a patch that should fix this.

Regards,
Daniel