What makes you drop a training plan?

I'm launching Lora here on Tuesday. It's a running coach for iPhone that rebuilds your week when you tell her what changed.

Before launch day I'd like to hear from runners. The last time you stopped following a plan, what broke it? An injury, work, a bad week of sleep, or the plan itself?

I built Lora around my own answers: long breaks, coming back with a body that didn't follow the schedule. I'm curious whether yours look the same, and what an app could have done differently.