When you enter a friends world it makes you download all installed addons that you do not have yet.  For some reason now when I try to download the addons the downloading process freezes and it does not continue.  After cancelling the download you are unable to enter the world until you clear the misc. cache and marketplace cache.  When I try to redownload everything it still does not work.  I think it happens with larger packs.  I also am pretty sure it is not my pc because I have a descent pc.  I have also encountered this bug on my laptop and my friend's sister encountered it to, however it was not as bad.