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