Changing the -3dB point of the closed loop gain. Normally the amplifier’s closed loop gain has a small capacitor across the feedback resistor. The increases the closed loop gain at high frequencies, but leaves it untouched within the audible band.
In this case, our Chief engineer, Bob Stadtherr, simply added some capacitance with his finger (we’re all small capacitors at heart). That was enough to eliminate the instability.