Spot the subtle error

#!/usr/bin/env python

# set up items, define is_type_x functions, etc

list_a = list_b = list_c = []

for item in items:
    if is_type_a(item):
        list_a.append(item)
    elif is_type_b(item):
        list_b.append(item)
    elif is_type_c(item):
        list_c.append(item)
    else:
        print 'wtf is this?',item

( answer inside cut )

Don't ask how long I was beating my head against this one.