Block indentation in Python mode often gives silly results. For example,
with 'emacs -q' create a Python buffer and type this:

def foo ():
    try:
        pass
    except:
        pass

Now 'C-x h TAB' gives something that is not valid at all:

def foo ():
    try:
        pass
except:
    pass

Note that if you go to the 'except:' line and press 'TAB', 'except:' is
indented back to its proper position. Further 'TAB's don't change this,
i.e. mode understands that it cannot place 'except:' at any other
indentation level.

Emacs version (just compiled):
GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.9) of
2014-10-26

Paul