mitchellh/vagrant — comment on commit 535bda2a4e14742f4bb948ed869a3cf749c553ae
File:  line 0
By semarj:

You are right. Good catch. The root of the problem is the case where dhcp gives you an IP address, the "ifconfig eth# down" doesn't fully take it down, and then "ifup" fails and the script stops. Is there some way to make line #42 be ifdown and not stop the script if it returns a nonzero exit code?