High Prandtl number convection possesses a square flow pattern that is steady and is apparently stable to infinitesimal disturbances. This pattern is unstable to finite‐amplitude disturbances, however, because a more chaotic (in time and space) spoke pattern of convection eats its way into the squares from the lateral boundaries. Experiments are described in which the breakup of the squares is initiated by dislocating one square in the middle of the apparatus with the use of a small, heated resistor. Once a critical heating rate and time is exceeded, the dislocation initiates a spoke cell which then systematically destroys neighboring square cells, resulting in the more chaotic spoke pattern. If the critical rate is not exceeded, the cell becomes severely deformed during the heating, but will relax back to a square convection cell after heating has ceased.