setting group contact cache to zero results in it being set to 5

According to the directions under the "Smart group cache timeout" option it says:

The number of minutes to cache smart group contacts. A value of '0' means the cache is emptied immediately when any contact is edited or a new one is added. If your contact data changes frequently, you may want to try setting this to a value of 5 minutes or so.

However, in practice, if you set that number to 0, CiviCRM will default to 5.

I'm not sure it's ever a good idea to set it to 0, however, it was useful for me while debugging CRM-12466.