Consider the following code:

from twisted.internet.defer import Deferred

d1 = Deferred()
d2 = Deferred()

def f1(result):
    print 'f1',

def f2(result):
    print 'f2',

def f3(result):
    print 'f3',

def fd(result):
    return d2

d1.addCallback(f1)
d1.addCallback(fd)
d1.addCallback(f3)

#/BLOCK====
d2.addCallback(f2)
d1.callback(None)
#=======BLOCK/

d2.callback(None)

This outputs what I would expect:

f1 f2 f3

However when I swap the order of the statements in BLOCK to

#/BLOCK====
d1.callback(None)
d2.addCallback(f2)
#=======BLOCK/

i.e. Fire d1 before adding the callback to d2, I get:

f1 f3 f2

I don't see why the time of firing of the deferreds should influence the callback execution order.
Is this an issue with Twisted or does this make sense in some way?