The problem occurs with the recent JDK 1.6.0_18 update and is mentioned in the Known Issues of the JDK release notes:
A flaw in the implementation of a card-marking performance optimization in the JVM can cause heap corruption under some circumstances. This issue affects the CMS garbage collector prior to 6u18, and the CMS, G1 and Parallel Garbage Collectors in 6u18. The serial garbage collector is not affected. Applications most likely to be affected by this issue are those that allocate very large objects which would not normally fit in Eden, or those that make extensive use of JNI Critical Sections (JNI Get/Release*Critical).
This issue will be fixed in the next Java SE 6 update.
Meanwhile, as a workaround to the issue, users should disable this performance optimization by