add Guido's reasoning behind running all tests twice

Makefile.pre.in

@@ -514,6 +514,13 @@ $(LIBRARY_OBJS) $(MODOBJS) Modules/$(MAINOBJ): $(PYTHON_HEADERS)
 ######################################################################
 
 # Test the interpreter (twice, once without .pyc files, once with)
+# In the past, we've had problems where bugs in the marshalling or
+# elsewhere caused bytecode read from .pyc files to behave differently
+# than bytecode generated directly from a .py source file.  Sometimes
+# the bytecode read from a .pyc file had the bug, somtimes the directly
+# generated bytecode.  This is sometimes a very shy bug needing a lot of
+# sample data.
+
 TESTOPTS=	-l
 TESTPROG=	$(srcdir)/Lib/test/regrtest.py
 TESTPYTHON=	$(RUNSHARED) ./$(BUILDPYTHON) -E -tt