I just finished writing a logstash-filter plugin with the most recent logstash-filter-example from github. When I went to install the plugin on logstash 2.3.2, I was unable to complete the installation because logstash 2.3.2 uses logstash-core-plugin-api 1.10.0 while the logstash-filter-example depends on logstash-core-plugin-api, "~> 2.0" (which I believe is only compatible with logstash 5.0.0-alpha3).
This seems like a version mismatch bug as the logstash 2.3 docs point to the master branch of https://github.com/logstash-plugins/logstash-filter-example.git.