Darlington Transistor Array Overheating and failure
With Vce-sat of maybe 1.6 V (at 500 mA), each darlington will be heated by ~0.8 W when run at maximum current. 

If all 8 darlingtons are active at once that's 6.4 W.

Junction-to-ambient thermal resistance is 66 C/W (for some assumed copper pattern on the board). 

So with with all 8 darlingtons active at 500 mA, you can expect a temperature rise of 422 degrees.

What this means in practice is: Don't operate more than one or two of the darlingtons at full current at the same time.