Most people make at least 8 or 9 of these in a new project, and this alone is a good reason to use a programming framework, unless you know what you're doing.

The problem is, sometimes, people who skip on frameworks, don't know what they're doing.
Or, as the Tao of Programming says:

There once was a master programmer who wrote unstructured programs. A novice programmer, seeking to imitate him, also began to write unstructured programs. When the novice asked the master to evaluate his progress, the master criticized him for writing unstructured programs, saying, ``What is appropriate for the master is not appropriate for the novice. You must understand the Tao before transcending structure.''

Anyway, for novices and masters alike, here's a great resource of common programming errors one should avoid while working on a project. Some are web oriented, but most are applicable in any environment.