See Docker issue #24834 for the more complete background. The short story is that if lxcfs is installed (which it is by default in Ubuntu 16.04) and you attempt to use the Docker daemon with user namespaces enabled, most likely you will end up with a confusing "container command could not be invoked error" like the one here:
# docker run --rm busybox date
docker: Error response from daemon: Container command 'date' could not be invoked..