Reading package lists... Error!
E: Dynamic MMap ran out of room. Please increase the size of APT::Cache-Limit. Current value: 25165824. (man 5 apt.conf)
E: Error occurred while processing xxx
E: Problem with MergeList /var/lib/apt/lists/xxx
W: Unable to munmap
E: The package lists or status file could not be parsed or opened.
Am unsure why this is happening, but suspect it has something to do with the cache size allocated for the repositories. A quick Google search revealed a debian bug1 that helped.
sudo echo 'APT::Cache-Limit "125829120";' > /etc/apt/apt.conf.d/30cache
Seems to have fixed the problem, a quick sudo apt-get update and sudo apt-get upgrade had me happily hacking away again.