After https://community.jboss.org/message/728205#728205:

The code in quesiton is here: https://github.com/arquillian/arquillian-core/blob/master/test/spi/src/main/java/org/jboss/arquillian/test/spi/ExceptionProxy.java#L225

Need a try catch around the ObjectOutputStream write part.. if it fails just write the empty byte array

The HTMLUnit Exception breaks the contract of Exception by not being serializable, and should be fixed as well, but I guess we can work around it.