The CVSTag plugin does not work for branches - it will only work for the head
due to the code listed below missing out specification of the branch (presuming
that there is one)

HUDSON/plugins/cvs-tag/src/main/java/hudson/plugins/cvs_tag/CVSTagPlugin.java -
starts at line 84

// cvs -d cvsRoot rtag -D toDate tagName modules
cmd.add(scm.getDescriptor().getCvsExe(), "-d", scm.getCvsRoot(), "rtag", "-D",
date, tagName, modules);
logger.println("Executing tag command: " + cmd.toStringWithQuote());

The code can be fixed by replacing it with this fragment which addes the branch
argument

ArgumentListBuilder cmd = new ArgumentListBuilder();

// BRANCH
// cvs -d cvsRoot rtag -r BRANCH_NAME -D tagDate tagName modules
if (scm.getBranch()!=null)

{ cmd.add(scm.getDescriptor().getCvsExe(), "-d", scm.getCvsRoot(), "rtag","-r", scm.getBranch(), "-D", date, tagName, modules); }

// NO BRANCH
// cvs -d cvsRoot rtag -D toDate tagName modules
else

{ cmd.add(scm.getDescriptor().getCvsExe(), "-d", scm.getCvsRoot(), "rtag", "-D", date, tagName, modules); }

logger.println("Executing tag command: " +
cmd.toStringWithQuote());