The FDA found several generic brands of blood pressure medication contaminated with high levels of a potential carcinogen called N-Nitrosodimethylamine. One of the generic brands contained levels 17 times higher than what the FDA considers safe.
This is a long but very interesting article that covers how generic brands are made abroad (mostly in China and India) and how the US FDA regulatory standards for generic drugs made in these other countries may not be as stringent as one would hope.