Suzanne Lucas, better known as the Evil HR Lady (she's very nice and not evil at all), did an interesting article about what employers are saying about former employees in references. Hint: It's not good.
Lucas says this: "Reference-checking firm Allison & Taylor estimates that 50 percent of their reference checks come back negative or lukewarm." If you want to chill your blood,
read the article
for some actual things employers have said about employees.