What's with all the outages and dropped Tweets?

Throughout the day, Twitter comes and goes. Some tweets get through, some don't. You might see an error, you might not. Even when the site appears to be functioning, you find out later you missed a friend's tweet. Additionally, SMS notifications are just as unreliable. Some come through, some don't. People are missing direct messages, only getting some of them. It's all very random. What is going on?