This tutorial is about a problem with Blender (up to version 2.76), which I consider its absolute worst failing: link-failure and the resulting loss of data. There are a number of ways around this problem, which I will cover in a 2nd tutorial, but in this one, I just want to focus on the problem and how you can tell that it's happening. Recovering from this is a pretty advanced topic, and if you've got anyone more experienced to turn to, this video will tell you if it's time to go ask for help!