The position of the highest level in a molecular potential is not well
described by the usual JWKB quantization formula. This is due to an e
ffect which is the mirror image for bound states of the quantum thresh
old behaviour for the continuum states. This new effect is described i
n the present letter and some consequences are briefly discussed. In p
articular, we discuss the probability distribution of the binding ener
gy of the highest vibrational level and show that its shape is strongl
y modified.