Simple Constant Current Circuit Acting Strange on Occasion
With 10 ohm load, and 5.1 ohms in emitter, the maximum current of 0.8 amps exceeds many small-signal device ratings, even when well heatsunk. (hard to extract heat from a plastic package).

At 0.4 amps (worse case power dissipation is usually at 1/2 Imax, 1/2 Vmax, the power dissipation of 0.4 amps* 6 volts (12v - 0.4 * 15 ohm) === 2.4 watts.

At high currents, the Ftau drops and the delay thru the device increases, putting the loop-stability at risk. To avoid this, add 100 ohm resistor in base AND add capacitor from emitter to opamp Vout. Value? 1uF.