This happens when your runtime classpath is different than your compile time classpath.
When your application was compiled, a class (named SomeInterface in your question) existed as a class.
When your application is running at compile time, SomeInterface exists as an interface (instead of a class.)
This causes an IncompatibleClassChangeError to be thrown at runtime.
This is a common occurence if you had a different version of a jar file on the compile time classpath than on the runtime classpath.