Running the pvcs.HistogramSelection test in valgrind, this appeared on both server processes.

==5854== Invalid read of size 8
==5854==    at 0x130AD9B8: vtkWeakPointer<vtkPythonInterpreter>::operator vtkPythonInterpreter*() const (vtkWeakPointer.h:92)
==5854==    by 0x130AC678: vtkPythonInterpreter::~vtkPythonInterpreter() (vtkPythonInterpreter.cxx:104)
==5854==    by 0x130AC745: vtkPythonInterpreter::~vtkPythonInterpreter() (vtkPythonInterpreter.cxx:110)
==5854==    by 0x72314C5: vtkObjectBase::UnRegisterInternal(vtkObjectBase*, int) (vtkObjectBase.cxx:232)
==5854==    by 0x7233B7E: vtkObject::UnRegisterInternal(vtkObjectBase*, int) (vtkObject.cxx:900)
==5854==    by 0x723138B: vtkObjectBase::UnRegister(vtkObjectBase*) (vtkObjectBase.cxx:189)
==5854==    by 0x72310D1: vtkObjectBase::Delete() (vtkObjectBase.cxx:134)
==5854==    by 0x12E96F41: vtkMatplotlibMathTextUtilities::~vtkMatplotlibMathTextUtilities() (vtkMatplotlibMathTextUtilities.cxx:143)
==5854==    by 0x12E96F9D: vtkMatplotlibMathTextUtilities::~vtkMatplotlibMathTextUtilities() (vtkMatplotlibMathTextUtilities.cxx:144)
==5854==    by 0x72314C5: vtkObjectBase::UnRegisterInternal(vtkObjectBase*, int) (vtkObjectBase.cxx:232)
==5854==    by 0x7233B7E: vtkObject::UnRegisterInternal(vtkObjectBase*, int) (vtkObject.cxx:900)
==5854==    by 0x723138B: vtkObjectBase::UnRegister(vtkObjectBase*) (vtkObjectBase.cxx:189)
==5854==  Address 0x3726c150 is 0 bytes inside a block of size 8 free'd
==5854==    at 0x4C2F24B: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5854==    by 0x130B00A7: __gnu_cxx::new_allocator<vtkWeakPointer<vtkPythonInterpreter> >::deallocate(vtkWeakPointer<vtkPythonInterpreter>*, unsigned long) (new_allocator.h:110)
==5854==    by 0x130AF9F3: __gnu_cxx::__alloc_traits<std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::deallocate(std::allocator<vtkWeakPointer<vtkPythonInterpreter> >&, vtkWeakPointer<vtkPythonInterpreter>*, unsigned long) (alloc_traits.h:185)
==5854==    by 0x130AEE1D: std::_Vector_base<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::_M_deallocate(vtkWeakPointer<vtkPythonInterpreter>*, unsigned long) (stl_vector.h:178)
==5854==    by 0x130ADDF9: std::_Vector_base<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::~_Vector_base() (stl_vector.h:160)
==5854==    by 0x130B09C1: std::vector<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::~vector() (stl_vector.h:425)
==5854==    by 0x7D21359: __cxa_finalize (cxa_finalize.c:56)
==5854==    by 0x130ABEF2: ??? (in /home/shawn/dev/paraview/build/lib/libvtkPythonInterpreter-pv5.1.so.1)
==5854==    by 0x4010C16: _dl_fini (dl-fini.c:235)
==5854==    by 0x7D20FE7: __run_exit_handlers (exit.c:82)
==5854==    by 0x7D21034: exit (exit.c:104)
==5854==    by 0x7D07836: (below main) (libc-start.c:325)
==5854==  Block was alloc'd at
==5854==    at 0x4C2E0EF: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5854==    by 0x130B016B: __gnu_cxx::new_allocator<vtkWeakPointer<vtkPythonInterpreter> >::allocate(unsigned long, void const*) (new_allocator.h:104)
==5854==    by 0x130AFB04: __gnu_cxx::__alloc_traits<std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::allocate(std::allocator<vtkWeakPointer<vtkPythonInterpreter> >&, unsigned long) (alloc_traits.h:182)
==5854==    by 0x130AF02F: std::_Vector_base<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::_M_allocate(unsigned long) (stl_vector.h:170)
==5854==    by 0x130AE016: std::vector<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::_M_insert_aux(__gnu_cxx::__normal_iterator<vtkWeakPointer<vtkPythonInterpreter>*, std::vector<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > > >, vtkWeakPointer<vtkPythonInterpreter> const&) (vector.tcc:353)
==5854==    by 0x130AD971: std::vector<vtkWeakPointer<vtkPythonInterpreter>, std::allocator<vtkWeakPointer<vtkPythonInterpreter> > >::push_back(vtkWeakPointer<vtkPythonInterpreter> const&) (stl_vector.h:925)
==5854==    by 0x130AC59F: vtkPythonInterpreter::vtkPythonInterpreter() (vtkPythonInterpreter.cxx:94)
==5854==    by 0x130AC51F: vtkPythonInterpreter::New() (vtkPythonInterpreter.cxx:90)
==5854==    by 0x12E96E85: vtkMatplotlibMathTextUtilities::vtkMatplotlibMathTextUtilities() (vtkMatplotlibMathTextUtilities.cxx:134)
==5854==    by 0x12E967D1: vtkMatplotlibMathTextUtilities::New() (vtkMatplotlibMathTextUtilities.cxx:59)
==5854==    by 0x12E9D48C: vtkObjectFactoryCreatevtkMatplotlibMathTextUtilities() (vtkRenderingMatplotlibObjectFactory.cxx:27)
==5854==    by 0x7235ECE: vtkObjectFactory::CreateObject(char const*) (vtkObjectFactory.cxx:492)