If a new master becomes elected, it will only have knowledge of the current state of the cluster. This can lead to a situation where tasks become lost but aren't properly killed. For instance:
1) A set of machines (perhaps a datacenter rack) lose network connectivity and their tasks are marked LOST by the master. However, they're still running.
2) Through a potentially unrelated situation, there is a master failover to a new master
3) The network connection to the machines come back up
4) These slaves never killed their tasks (and they shouldn't if they can't talk to a master)
5) Tasks stay running and aren't killed, taking up resources and running outside the scope of the new master