Save the following as a .py file and open in an 'emacs -q':
if True:
x ()
if True:
Put the point on the second 'if', i.e. line 3, column 4.
* First press of DEL (Backspace) does nothing (bug 1);
* Second press dedents the line --- ok;
* Press C-/ to undo line dedention and so revert the buffer to original text;
* Now third press of DEL _indents_ the line, even though it makes no sense in Python at all (bug 2).
The buggy function is 'python-indent-dedent-line-backspace'.
Paul