It is easy in theology to understand the thrust of scripture and then to begin to see implications the bible does not see. The bible is very clear in its teaching and often qualifies statements.
As Christians it is easy to often grasp the thrust of the teaching and to miss all the nuance. We may see that God is love and thus forget that God has a righteous anger. We see the concept of love and begin to play it out in a way which makes love and righteous anger incompatible.
The examples of these sorts of issues is profoundly large in scale. It is often that a preacher will grasp the thrust of a biblical text but take the implications too far. Most theological errors of minor and serious degree take truths in scripture and begin to take the implications far past where scripture takes the implications and at times to the point of denying other principles in scripture (although it is rarely stated so crassly).
In theology we can often most easily notice an error in a system when we see certain elements become so absolutized they seem to push other elements taught in scripture out.
Good theology by nature of the discipline is an attempted approximation of the teachings of God through his inspired writers as found in the old and new testaments. To the degree that theology fails to approximate scripture or even worse contradicts scripture it is to that degree that the theology fails.