RobotFileParser.parse() contains the lines

                elif line[0] == "disallow":
                    if state != 0:
                        entry.rulelines.append(RuleLine(line[1], False))
                        state = 2
                elif line[0] == "allow":
                    if state != 0:
                        entry.rulelines.append(RuleLine(line[1], True))

with no 'state = 2' in the 'allow' part.
This causes different behaviour depending on whether the file ends with
'allow' or 'disallow', or an empty line.

Those lines were taken from revision 65118.  My Python 2.5 sources are
similar.  I have not checked others.