Here's what the problem looks like:
Error: java.lang.ClassNotFoundException: org.apache.mahout.math.Vector
First the good news: it is fixed upstream:
MAHOUT-680. The root cause of the problem is the fact that /usr/lib/mahout/bin/mahout constructs HADOOP_CLASSPATH in such a way that it puts mahout-examples-0.5-job.jar after mahout-core-0.5-job.jar. That trips up job submission code into putting mahout-core-0.5-job.jar into distributed cache instead of mahout-examples-0.5-job.jar (which has the missing class definitions).
A proposed workaround to sustain us for as long as
MAHOUT-680 hasn't been put into any release yet is to manually initialize HADOOP_CLASSPATH with mahout-examples-0.5-job.jar at the level of /usr/bin/mahout. This is ugly but it works.