Numerous times. The worst was the time it when the only employee maintaining a little-used product was fired for sexual harassment. I transferred to the position not knowing anything about the product or the lack of developer knowledge. The support person gave me a walk-through of the functionality as did the QA person. I spent about four months figuring out the build system and the code. For the code, it was just a matter of reading, reading, reading until I came to understand it. Making matters worse was that a massive refactor of the had been started but not completed. It was very stressful.