There are events that may affect more than one machine which are not netsplits.

e.g. an ISP with common users experiences an outage, an IRC client with common users has a bug, common users within the same time zone have automated system updates run at the same time, the IRC server experiences an upstream network disruption affecting only some routes, a regional power outage occurs, a hosted bouncer service with common users has an outage, etc, etc, etc...