Incremental builds are completely broken in the Mustang master as of 2005-07-26.
A simple recipe for failure is:

cd make
rm -rf ../build
make all # succeeds
touch ../src/share/classes/java/util/HashMap.java
make all # fails

The last make fails with:

# Java sources to be compiled: (listed in file ../../../build/solaris-i586/tmp/java/java.lang/java/.classes.list)
../../../src/share/classes/java/util/HashMap.java
# Running javac:
/usr/jdk/jdk1.5.0_01/bin/javac -J-client -J-Xmx128m -classpath ../../../build/solaris-i586/classes -bootclasspath ../../../build/solaris-i586/lib/jce.jar:../../../build/solaris-i586/lib/jsse.jar -sourcepath ../../../build/solaris-i586/gensrc:../../../src/solaris/classes:../../../src/share/classes -d ../../../build/solaris-i586/classes -encoding ascii -source 1.5 @../../../build/solaris-i586/tmp/java/java.lang/java/.classes.list
../../../src/share/classes/java/util/HashMap.java:102: cannot access java.util.AbstractMap
bad class file: ../../../build/solaris-i586/classes/java/util/AbstractMap.class
class file has wrong version 50.0, should be 49.0
Please remove or make sure it appears in the correct subdirectory of the classpath.
    extends AbstractMap<K,V>
            ^
1 error
make[2]: *** [.compile.classlist] Error 1
make[2]: Leaving directory `/u/martin/ws/MASTER/make/java/java'

The problem appears to only occur when rebuilding "core" classes, perhaps only
for classes needed by the bootstrap javac.