FactoryGirls randomly fails with 'Factory not registered', why?

FactoryGirls randomly fails with 'Factory not registered', why?

I've some tests that randomly fail, approx. 20% of times. It means that
WITHOUT changing the code, each time that I run the tests 1 time out of 5
will fail with "Factory not registered" error. It's very weird.. :(
This is the consone output:
Failures:
1) Unit#new_from_string returns factor for metric conversions
Failure/Error: FactoryGirl.create :taza
ArgumentError:
Factory not registered: taza
# ./spec/models/unit_spec.rb:29:in `block (2 levels) in <top
(required)>'
Finished in 0.29619 seconds
4 examples, 1 failure
Failed examples:
rspec ./spec/models/unit_spec.rb:22 # Unit#new_from_string returns factor
for metric conversions
Randomized with seed 61727
And this is the code:
file: "unit_spec.rb"
require 'spec_helper'
describe Unit, "#new_from_string" do
it "parses the string and returns a Unit object" do
[some tests...]
FactoryGirl.find_definitions
u = FactoryGirl.create :taza
FactoryGirl.create :tbsp
[tests...]
end
it "returns factor for metric conversions" do
[tests not involving factory girl...]
# the following is line 29, that fails
FactoryGirl.create :taza
[tests...]
end
end
file "spec/factories/units.rb":
FactoryGirl.define do
factory :taza , :class => CustomUnit do
singular 'taza'
plural 'tazas'
physical_type Unit::VOLUME
equivalence_factor 200
equivalence_unit 'ml'
end
[other factories...]
end