marked unimplemented_gradient as unsafe to use

theano/gradient.py

 """Driver for gradient calculations."""
 
-__authors__   = "James Bergstra"
+__authors__   = "James Bergstra, Ian Goodfellow"
 __copyright__ = "(c) 2011, Universite de Montreal"
 __license__   = "3-clause BSD License"
 __contact__   = "theano-dev <theano-dev@googlegroups.com>"
@@ -123,6 +123,8 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
 
 def unimplemented_grad(op, x_pos, x):
     """
+    DO NOT USE. Remove this function after all usage of it has been removed from theano.
+
     Return an un-computable symbolic variable of type `x.type`.
 
     If any function tries to compute this un-computable variable, an exception
@@ -130,5 +132,14 @@ def unimplemented_grad(op, x_pos, x):
     `x_pos`'th input of `op` has not been implemented.
 
     """
+
+    raise Exception("""
+                        unimplemented_grad is not a safe function to use.
+                        It depends on catching errors at the run-time of a theano function.
+                        However, it could be removed by the optimization during the compilation
+                        of the theano function, for example, if it is multiplied by 0. This
+                        results in theano functions returning 0 for gradients that are actually
+                        undefined. """)
+
     msg = '%s.grad not implemented for input %i'%(op, x_pos)
     return Raise(msg=msg)(x)