  We discuss the extraction of form factors from three-point sum rules making
use of harmonic-oscillator model, where we derive the exact expression for the
relevant correlator. We determine the form factor of the ground state by the
standard procedures adopted in the method of sum rules, and compare the
obtained results with the known exact values. We show that the uncontrollable
uncertainty in the extracted value of the form factor is typically much larger
than that for the decay constant. In the example considered, we find the
uncontrolled systematic error in the extracted form factor to exceed the 10%
level.
