py.test hangs after showing test results

I'm using py.test to run a bunch of tests. The tests seem to pass, but the process never terminates:

===== test session starts =====

platform win32 -- Python 2.7.3 -- pytest-2.3.4

collected 179 items

common/tests/test_bar.py ...............

common/tests/test_foo.py .......

....

===== 159 passed, 20 skipped in 98.58 seconds =====

<-- the prompt never gets back -->

Any ideas what can cause this and how to debug?

EDIT

@hpk42 is right - it was a non-daemon thread that never terminated.