Our minds are pretty chaotic systems, which is why following the middle way is so difficult. It's so easy for a chaotic system to get knocked out of equilibrium, to veer off to the left, to veer off to the right. Staying in the middle is difficult; it requires a lot of balance.
So it's no wonder that when psychologists describe the workings of the mind, they tend to use fluid mechanics as their main analogy. In other words, the mind acts like water: the way water swirls around and forms eddies, the way it gets pushed down here, goes underground, and comes out over there. Sometimes it builds up into huge waves. I was reading the other day about enormous waves, called rogue waves, that suddenly form out in the ocean. Things come together just right -- this little wave adds on to that little wave -- and all of a sudden they build into enormous rogue waves that flow through the ocean and then after a while, mysteriously dissipate. It's the same with the mind. Sometimes rogue waves come crashing through the mind and you wonder where they came from.
It's easy for tiny little things to set them off. This is why we have to be careful in our practice. Don't regard the little details or little distractions as totally unimportant. [Meditation Teacher] Ajaan Mun used to say that it's very rare that a whole tree gets into your eye, but sometimes just a little sawdust can get into your eye and blind you. Sometimes the little things are the ones that set you off, so you have to be careful.
On the other hand, though, you have to be confident that even if the mind does get knocked off balance you can bring it back. Otherwise the practice would be full of fear all the time -- afraid of tipping off too far to the left, too far to the right, toppling upside down. There's only one way of learning what the balance point is, and that's through experimenting. [...]
Don't view a mistake as a really bad thing that you have to make up for afterwards. Use it as a learning experience. When you approach everything as a learning experience then no meditation is wasted. Every meditation becomes an opportunity to learn. It's just a question of whether you take that opportunity or not.
--Thanissaro Bhikkhu, from "Meditations 2"