The problem can occur if the provisional patent application were to be found in court not to support the "full patent" filed with a lawyer. For example, if the "full patent" includes 10 pages of explanatory text, and 10 drawings, and the provisional patent application filed by the inventor only contains 1 paragraph of explanation, and 1 drawing, aspects of the "full patent" might not be apble to claim priority to (or be suported by)the provisional application.