  Cold atom interferometers use guiding potentials that split the wave function
of the Bose-Einstein condensate and then recombine it. We present theoretical
analysis of the wave function recombination instability that is due to the weak
nonlinearity of the condensate. It is most pronounced when the accumulated
phase difference between the arms of the interferometer is close to an odd
multiple of PI and consists in exponential amplification of the weak ground
state mode by the strong first excited mode. The instability exists for both
trapped-atom and beam interferometers.
