diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
index d4d6ce6b2bc4..1e43f4b6f3e2 100644
--- a/lib/matplotlib/cbook/__init__.py
+++ b/lib/matplotlib/cbook/__init__.py
@@ -1168,9 +1168,9 @@ def __setitem__(self, k, v):
 
 class Stack(object):
     """
-    Implement a stack where elements can be pushed on and you can move
-    back and forth.  But no pop.  Should mimic home / back / forward
-    in a browser
+    Stack of elements with a movable cursor.
+
+    Mimics home/back/forward in a web browser.
     """
 
     def __init__(self, default=None):
@@ -1178,62 +1178,65 @@ def __init__(self, default=None):
         self._default = default
 
     def __call__(self):
-        """return the current element, or None"""
+        """Return the current element, or None."""
         if not len(self._elements):
             return self._default
         else:
             return self._elements[self._pos]
 
     def __len__(self):
-        return self._elements.__len__()
+        return len(self._elements)
 
     def __getitem__(self, ind):
-        return self._elements.__getitem__(ind)
+        return self._elements[ind]
 
     def forward(self):
-        """move the position forward and return the current element"""
-        n = len(self._elements)
-        if self._pos < n - 1:
-            self._pos += 1
+        """Move the position forward and return the current element."""
+        self._pos = min(self._pos + 1, len(self._elements) - 1)
         return self()
 
     def back(self):
-        """move the position back and return the current element"""
+        """Move the position back and return the current element."""
         if self._pos > 0:
             self._pos -= 1
         return self()
 
     def push(self, o):
         """
-        push object onto stack at current position - all elements
-        occurring later than the current position are discarded
+        Push *o* to the stack at current position.  Discard all later elements.
+
+        *o* is returned.
         """
-        self._elements = self._elements[:self._pos + 1]
-        self._elements.append(o)
+        self._elements = self._elements[:self._pos + 1] + [o]
         self._pos = len(self._elements) - 1
         return self()
 
     def home(self):
-        """push the first element onto the top of the stack"""
+        """
+        Push the first element onto the top of the stack.
+
+        The first element is returned.
+        """
         if not len(self._elements):
             return
         self.push(self._elements[0])
         return self()
 
     def empty(self):
+        """Return whether the stack is empty."""
         return len(self._elements) == 0
 
     def clear(self):
-        """empty the stack"""
+        """Empty the stack."""
         self._pos = -1
         self._elements = []
 
     def bubble(self, o):
         """
-        raise *o* to the top of the stack and return *o*.  *o* must be
-        in the stack
-        """
+        Raise *o* to the top of the stack.  *o* must be present in the stack.
 
+        *o* is returned.
+        """
         if o not in self._elements:
             raise ValueError('Unknown element o')
         old = self._elements[:]
@@ -1249,7 +1252,7 @@ def bubble(self, o):
         return o
 
     def remove(self, o):
-        'remove element *o* from the stack'
+        """Remove *o* from the stack."""
         if o not in self._elements:
             raise ValueError('Unknown element o')
         old = self._elements[:]
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
index 7d72a291d949..e8ad7f56bd0f 100644
--- a/lib/matplotlib/figure.py
+++ b/lib/matplotlib/figure.py
@@ -1792,11 +1792,8 @@ def gca(self, **kwargs):
             # if the user has specified particular projection detail
             # then build up a key which can represent this
             else:
-                # we don't want to modify the original kwargs
-                # so take a copy so that we can do what we like to it
-                kwargs_copy = kwargs.copy()
                 projection_class, _, key = process_projection_requirements(
-                    self, **kwargs_copy)
+                    self, **kwargs)
 
                 # let the returned axes have any gridspec by removing it from
                 # the key
@@ -1806,6 +1803,15 @@ def gca(self, **kwargs):
                 # if the cax matches this key then return the axes, otherwise
                 # continue and a new axes will be created
                 if key == ckey and isinstance(cax, projection_class):
+                    cbook.warn_deprecated(
+                        "3.0",
+                        "Calling `gca()` using the same arguments as a "
+                        "previous axes currently reuses the earlier "
+                        "instance.  In a future version, a new instance will "
+                        "always be created and returned.  Meanwhile, this "
+                        "warning can be suppressed, and the future behavior "
+                        "ensured, by passing a unique label to each axes "
+                        "instance.")
                     return cax
                 else:
                     warnings.warn('Requested projection is different from '
diff --git a/lib/matplotlib/projections/__init__.py b/lib/matplotlib/projections/__init__.py
index 1e423420b0b6..9e01b4bb4295 100644
--- a/lib/matplotlib/projections/__init__.py
+++ b/lib/matplotlib/projections/__init__.py
@@ -67,15 +67,11 @@ def get_projection_class(projection=None):
 
 def process_projection_requirements(figure, *args, **kwargs):
     """
-    Handle the args/kwargs to for add_axes/add_subplot/gca,
-    returning::
+    Handle the args/kwargs to add_axes/add_subplot/gca, returning::
 
         (axes_proj_class, proj_class_kwargs, proj_stack_key)
 
-    Which can be used for new axes initialization/identification.
-
-    .. note:: **kwargs** is modified in place.
-
+    which can be used for new axes initialization/identification.
     """
     ispolar = kwargs.pop('polar', False)
     projection = kwargs.pop('projection', None)
@@ -94,7 +90,7 @@ def process_projection_requirements(figure, *args, **kwargs):
         kwargs.update(**extra_kwargs)
     else:
         raise TypeError('projection must be a string, None or implement a '
-                            '_as_mpl_axes method. Got %r' % projection)
+                        '_as_mpl_axes method. Got %r' % projection)
 
     # Make the key without projection kwargs, this is used as a unique
     # lookup for axes instances