When mongod detects an environment using OpenVZ, it logs a warning:

[initandlisten] ** WARNING: You are running in OpenVZ. This is known to be broken!!!

This is a holdover from much older versions of OpenVZ - this appeared to first be reported more than 2.5 years ago in SERVER-1121. All reasonably modern versions of OpenVZ (I haven't tested every version, but haven't seen this happen with any CentOS 6+ kernel) work well with MongoDB, and the outdated warnings scare off potential users. This should probably be eliminated, or at least modified to recommend using a current version instead of a blanket "broken" statement.