At PyCon '08, I gave a talk on testing and the OLPC project where I referred to the "Testing Death Spiral". My accompanying slide, which aimed to be simple rather than comprehensive, had this scenario:

1.

Write a bunch of code & manually test it.

(Good so far.)
2.

Start adding features over here.
3.

Watch code break over there.
4.

Rinse, lather, repeat

(Where do you think this ends?)