I tried two combinations. 9 + 15 and 8 + 14. In both cases the 1's digit displayed incorrectly. It didn't display a wrong number, just a glitched shape. I think you may have the wires arranged the wrong way so that it is turning on the wrong segements in that digit.
EDIT: Then again I tried 3 + 11 and both digits coded properly for 14. From what I can see it is not a problem with the addition, as after going through the adder, I noticed that it was indeed 24 (this is for 9 + 15) I think there is a problem with the wiring to the segments (the green-bluish chip near the end) you probably accidentally made it wire to the wrong semgement, so you should just go through solutions 1-32 and make sure you did it right.