Description of problem: If one of the host's network interfaces has the same subnet/broadcast as the host-only virtualbox network, there will be problems creating and accessing the docker-machine vm(s). They manifest themselves as "no route to host" errors during creation (visible with the --debug flag). The vm will actually launch and look like it is running, but then subsequent docker commands against it will get timeout errors.
I came across this because one of the wifi networks I use has 192.168.99.1/24 as its CIDR, which is the same subnet and broadcast that docker-machine uses by default for the virtualbox host-only network it uses.
It appears virtualbox will happily create a host-only network that conflicts with a host interface, and not print any errors. It just won't set up routing for it. They probably allow this because you can still set up a manual static route and get it to work - maybe for some advanced use-case.
For docker-machine, though, it just results in an unusable environment with cryptic errors.

docker version:

Client:
 Version:      1.10.0
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   590d5108
 Built:        Thu Feb  4 18:18:11 2016
 OS/Arch:      darwin/amd64

Server:
 Version:      1.10.0
 API version:  1.22
 Go version:   go1.5.3
 Git commit:   590d5108
 Built:        Thu Feb  4 19:55:25 2016
 OS/Arch:      linux/amd64

docker info:

Containers: 7
 Running: 0
 Paused: 0
 Stopped: 7
Images: 210
Server Version: 1.10.0
Storage Driver: aufs
 Root Dir: /mnt/sda1/var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 238
 Dirperm1 Supported: true
Execution Driver: native-0.2
Logging Driver: json-file
Plugins: 
 Volume: local
 Network: host bridge null
Kernel Version: 4.1.17-boot2docker
Operating System: Boot2Docker 1.10.0 (TCL 6.4.1); master : b09ed60 - Thu Feb  4 20:16:08 UTC 2016
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 7.79 GiB
Name: default
ID: GZCU:NH4C:QNFU:NQQB:KB55:ULRS:HKZW:AMMR:PSBI:FMFG:SMV4:S2JM
Debug mode (server): true
 File Descriptors: 11
 Goroutines: 22
 System Time: 2016-02-16T18:01:35.563863286Z
 EventsListeners: 0
 Init SHA1: 
 Init Path: /usr/local/bin/docker
 Docker Root Dir: /mnt/sda1/var/lib/docker
Labels:
 provider=virtualbox

docker-machine version:
docker-machine version 0.6.0, build e27fb87

uname -a:
Darwin mac.local 15.3.0 Darwin Kernel Version 15.3.0: Thu Dec 10 18:40:58 PST 2015; root:xnu-3248.30.4~1/RELEASE_X86_64 x86_64

Environment details (AWS, VirtualBox, physical, etc.): physical laptop, macbook pro

How reproducible:
Basically put your host on a network with a subnet of 192.168.99, and try to create a dockermachine vm with the defaults. Or try to create a virtualbox machine vm with the --virtualbox-hostonly-cidr set to your router's CIDR.

Steps to Reproduce:

1. determine the CIDR of your host's network. (use an online calculator, or whatever). Let's say it's 192.168.0.1/24
2. run docker-machine create --driver virtualbox --virtualbox-hostonly-cidr "192.168.0.1/24" cidrfail

Actual Results:
The create command will print the warning:
Error creating machine: Error checking the host: Error checking and/or regenerating the certs: There was an error validating certificates for host "192.168.0.100:2376": dial tcp 192.168.0.100:2376: connect: no route to host
N.B: with default values, you won't see this error because the quickstart terminal hides it.

docker-machine ls will print the following:

NAME       ACTIVE   DRIVER       STATE     URL                          SWARM   DOCKER    ERRORS
cidrfail   -        virtualbox   Running   tcp://192.168.0.100:2376             Unknown   Unable to query docker version: Get https://192.168.0.100:2376/v1.15/version: dial tcp 192.168.0.100:2376: connect: host is down
default    *        virtualbox   Running   tcp://192.168.200.100:2376           v1.10.0

If you're using the default vboxnet0 CIDR, the URL will actually be tcp://192.168.99.100:2376

Additional Info:
For the default case, you can see the lack of route to the vboxnet0 interface in the routing table. It looks like virtualbox maybe creates a second route to 192.168.99.1, but assigns it to interface en0 rather than vboxnet0

    Internet:
    Destination        Gateway            Flags        Refs      Use   Netif Expire
    default            192.168.99.1       UGSc           46        0     en0
    ...
    192.168.99         link#4             UCS             9        0     en0
    192.168.99.1/32    link#4             UCS             2        0     en0
    192.168.99.1       dc:9f:db:2b:89:39  UHLWIir        46       61     en0   1197
    192.168.99.100     link#4             UHLWIi          1        3     en0