You know you're screwed when the sample you've expended a lot of time and effort to prepare and purge develops a mysterious green precipitate.
Scientific research provides an endless abundance of anecdotes, but they can all be covered under this blanket statement:
You know you're screwed if you're a grad student.
(Why yes, I do spend too much time reading
http://www.phdcomics.com/comics/comics.php)