I am also receiving the "superclass mismatch for class Error" upon startup, as well.
It seems to happen randomly, too. Restarting logstash once or multiple times seems to allow logstash to finally start properly.
LS Version = 1.3.3 (via elasticsearch repo RPMs).
OS = Amazon Linux AMI 2013.09 (RHEL 6 variant)
Instance Type = c3.large (2 vCPU, 3.75 GB RAM)
Java Max Heap Size = 1875m
My two sanitized conf files are attached and exist as /etc/logstash/conf.d/