When you connect to mongo before forking, a segfault occurs on CentOS 7.  This does not happen with 2.10.4 and seems to have been introduced in 2.11.0.

require 'mongo'

Mongo::Client.new('mongodb://127.0.0.1:27017/evil')

Process.fork do
  Mongo::Client.new('mongodb://127.0.0.1:27017/evil')
end

A workaround is to close the connection before forking, but that is problematic in our case.