HTTPConduit in 2.6.x does not correctly close the stream for oneway call that results in HTTP 500. Instead of closing the error stream, it tries to close the input stream. And this leads to a confusing exception message and might potentially lead to other issue.
This problem might be related to the socket already closed exception that has been occasionally observed for CXF 2.6.x with recent jdk1.6.0_38 and 39 but not with earlier jdk versions.
Note that this problem is not relevant for 2.7.x and 2.8.x, as HTTPConduit is differently implemented in 2.7.x.