I've had several programming jobs. Each one with 20-50 developers, project going on for 3-5 years.

Every time it's the same. Some programmers are bright, some are average. Everyone has their CS degree, everyone read design patterns. Intentions are good, people are trying hard to write good code but still after a couple of years the code turns into spaghetti. Changes in module A suddenly break module B. There are always these parts of code that no one can understand except for the person who wrote it. Changing infrastructure is impossible and backwards compatibility issues prevent good features to get in. Half of the time you just want to rewrite everything from scratch.

And people more experienced than me treat this as normal. Is it? Does it have to be? What can I do to avoid this or should I accept it as a fact of life?

Edit: Guys, I am impressed with the amount and quality of responses here. This site and its community rock!