Focus on what you did wrong. Suggest that perhaps the person in the previous position was not trained themselves, or they quit without notice before you took the job.

Never say that "well, so and so goofed up and didn't tell me any better." Keep it positive!

"In a previous position, I was thrown into a role where the person who held it last had quit with no notice. The company needed to scramble to find an appropriate person to fill the position. There was quite a learning curve, and I did several things wrong to start. I asked supervisors and co-workers questions, but in many cases, they didn't even know the proper way to handle a given task, as it was not part of what they did daily. With a lot of support and understanding from my team, I was able to work out everything I didn't know pretty quickly! By the time I left, I was able to train my replacement."