when indent is set to 3 spaces, if you acidentally add a space after the normal indents, backspace does not delete the extra space, but instead deletes the default indent... which does not match any other indent level.
as a result , the only way to fix indentation errors is to go all the way back, and start fresh. 

def a():
   if b:
      #line auto-indented here
       c()# whoops, i have an extra space, better backspace

def a():
   if b:
      #line auto-indented here
    c()# after backspace once..  aligned to nothing: s/b aligned to previous indent

def a():
   if b:
      #line auto-indented here
 c()# after backspace twice.. also aligned to nothing