With gentoo, if you allocate let's say 20G to / on ext4, then you can quite easily run into this issue.
/usr/src/linux will use about 30% of the space and 10% of the inodes.
/var/db/repos/gentoo will use about 4% of the space and 10% of the inodes.
Next you clone the firefox-source hg repo, which will use about 15% of the space and 80% of the inodes.