tl;dr: boot Linux with maxcpus=2 to reproduce this failure in about one in four Maven builds.
In hundreds of scripted builds, many other failures were also seen, all related to EPSG code lookup. The core finding of this investigation is an NPE at org.hsqldb.Database.connect(Database.java:384). I suspect a synchronisation problem with EPSG database access.
The super-long version:
Attached patch for gt-epsg-hsql changes ImageMosaicReader.getReader to print a stack trace rather than swallowing exceptions.
Attached hsqldb-2.3.0.jar has been rebuilt with debugging information (needed Oracle JDK 7 to build). This gives us line numbers in stack traces and allows breakpoints in source.
Linux booted with maxcpus=2.
Building gs-main with mvn -o clean install.
After several builds the following is seen: