I myself have attended many lessons without practicing, and I have reaped the karma of doing so by having to teach many students who came to lessons completely unprepared. I was determined to make their (usually their parents') money worth the lesson though, and over the years, I have been coming up with different things to teach students when they haven't practiced. I find it to be a fun challenge, and it pushes me to think outside the normal "lesson schedule" box (tone, technique, etude, repertoire, repeat...).
I discovered that these were perfect lessons to discuss the history of the flute, the current development of the flute, things they observe about the music world that they have questions about, our experiences with practicing and performing, and walking them through efficient ways to practice so they don't feel overwhelmed by the time they leave the lesson. Most students who come to lessons without having practiced usually feel powerless, helpless, and like generally horrible humans, but I want to empower them and give them as many tools to succeed despite the odds of not having enough time and energy to practice.
Sure, you should practice, but sometimes you just can't. There's nothing wrong with that--we're only human after all. You just need to pick yourself back up after you regain your energy (and hopefully some free time). :)