After adding a few extra repositories to /etc/apt/sources.list I then ran an apt-get update and came accross the following error:

Fetched 8399kB in 5s (1427kB/s)
Reading package lists... Error!
E: Dynamic MMap ran out of room
E: Error occurred while processing umbrello (NewVersion1)
E: Problem with MergeList [...]
E: The package lists or status file could not be parsed or opened.

The reason being is that the disk cache used by apt when parsing packages is now too small to handle the number of packages available.

On debian, I created a new file at /etc/apt/apt.conf.d/99user adding the following line:

APT::Cache-Limit "20000000";

Don’t forget the trailing semicolon (;).

This fix is for any distro using apt-get, like debian, ubuntu etc.