ClassCastException related to IDEA specific classes when running outside of IntelliJ

I've been getting the occasional ClassCastException when running a project that's related to what seems to be internal classes of IntelliJ.

The project is a Java Webstart client (yes, legacy code) that's launched from a WAR file inside an EAR file that's built using an ANT job (did I say this is legacy code yet?) that was run from within IntelliJ.

The EAR itself is deployed on a JBoss server that's not connected to IntelliJ in any way.

"Exception "java.lang.ClassNotFoundException: com/intellij/codeInsight/editorActions/FoldingData"while constructing DataFlavor for: application/x-java-jvm-local-objectref; class=com.intellij.codeInsight.editorActions.FoldingData
Exception "java.lang.ClassNotFoundException: com/intellij/codeInsight/editorActions/FoldingData"while constructing DataFlavor for: application/x-java-jvm-local-objectref; class=com.intellij.codeInsight.editorActions.FoldingData"

It doesn't seem to cause any trouble inside the application, but it's weird. As the production deployment archives are created by a separate build process running outside of IntelliJ it shouldn't be a problem there either.

Any idea why this may be (occasionally) happening?