Unnecessary Words

Unnecessary words, like missing words, tend to result from careless proofreading:

I believe if schools or the government don't step in and do something about obesity in America today, I think we will be in trouble.

Including both I believe and I think in this sentence is repetitious.  Leaving either expression out would result in a clearer sentence:

I believe if schools or the government don't step in and do something about obesity in America today, we will be in trouble.

If schools or the government don't step in and do something about obesity in America today, I think we will be in trouble.

In fact, some people might argue that since the sentence is the writer's prediction, not a statement of fact, both I believe and I think are unnecessary:

If schools or the government don't step in and do something about obesity in America today, we will be in trouble.

While we would not mark a sentence with one unnecessary I think/I believe as an error, papers are usually more powerful when they do not contain unnecessary hedges.