added more information to comment.

theano/tensor/opt.py

@@ -329,7 +329,8 @@ class ShapeFeature(object):
         shape inference is 1) to mark shared inputs as having a particular shape,
         either via a .tag or some similar hacking; and 2) to add an optional Param() argument
         to promise that inputs will have a certain shape (or even to have certain shapes in
-        certain dimensions).
+        certain dimensions). We can't automatically infer the shape of shared variable as
+        they can change of shape during the execution by default.
 
 
     """