Beginning in 2.5.0, confirmed with Ruby 2.5.0 and Ruby 2.4.1, when requiring mong/error, an exception will be raised that Forwardable isn't defined, breaking backward compatibility.

This is because the class lib/mongo/error/operation_failure.rb uses Forwardable but does not require it.

Searching the codebase for all instance of Forwardable, it appears as though this may be a systemic issue.