"It's yoga practice, not yoga perfect." This is a phrase that I've heard from teachers numerous times, and it really hits home. There is no conceivable way we could achieve complete perfection in class or even in an individual posture.
A comparison I like to draw is with music, and rock music in particular. If you listen to any band perform live, you'll hear all kinds of minor errors, whether it's the rhythm speeding up or slowing down slightly, wrong notes, or instruments going out of tune. In it's extreme, this would be terrible, and the song wouldn't even be recognizable. But assuming the basic structure of the song is intact, the various errors actually add to the enjoyment and experience. What your witnessing is life, and real living breathing human beings, and that's what makes art (and yoga) work.
Frankly, yoga wouldn't even be yoga if the purpose was to do perfect postures, live a perfect life, have perfect concentration, etc. In a way, it's a gradient scale towards achieving more awareness of yourself and life. There is no ultimate goal, no perfect state to achieve. That's honestly a large part of the appeal for me. Yes, I'm pushed to improve my practice, but there's no impossible standard I'm trying to reach or an unattainable plateau. I just learn from the mistakes and try to do better next time, but the process is where the real gain for me comes from.