This pfSense bug has been present since 2.2 and is still present at the time of writing this. It occurs when using limiters on a pair of pfSense servers set up in a HA cluster. When connections are received which match the firewall rule you have put in place to force the traffic into the relevant limiter, those states fail to correctly sync and pfSense will experience extremely high load values. You will see the message pfsync_undefer_state: unable to find deferred state on the console and in logs. pfSense may also become inaccessible over HTTP and SSH.
The “fix” is to disable all synchronization of state. To do this, go to System -> High Avail. Sync and untick the Synchronize states box. You will need to do this on all nodes in the HA cluster.
It should be noted that this isn’t a particularly good fix. If states do not synchronize between nodes in the cluster, TCP connections will be (not very cleanly) terminated and must be re-established. Some applications may not respond too well to this.