With system 228 (actually 226, but we never included that), the system is per
default configured to limit the max tasks on a user session to 4096

wget seems to very easily be able to exhaust this limit on its own when using
the -mirror parameter.

before starting wget --mirror:

cat /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.current 
389

after running wget --mirror for a couple seconds:
cat /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.current 
1195

(the number is increasing constantly when checking the file, until you can't do
anything with your system for 'resource temporary unavailable. wget keeps on
running)

As soon as wget terminates (on it's own or being killed) the pids.current gets
back to the initial value.