The best way to avoid sequencing mistakes is to make both ends of each side the same way IE, Pin first. Then make the front and back with the space first. This is why I label the front and back "A" and the two ends "B" It doesn't matter what you label them as long as you keep them in pairs. Doing it this way also allows you to put the best side of each board facing out when it's time for you to glue them up, because both ends of each pair will be cut the same.
If you make each board pin first on one end and space first on the other end, I can almost guarantee that you will foul something up and the box won't go together.
After cutting both ends of the "A" sides and are about to begin cutting the "B" sides, Take one of the "A" sides and place the first space over the pins in the I-Box Jig with the rest of the board and pins to the right. Then place the end of the "B" board in the jig to the left of the "a" board, but tight up against it. Doing this will space the "B" board correctly and your first cut in it will begin at the right place. Once this first cut is made, remove the "A" board and complete cutting the end of the "B" board. Do this same procedure when you begin cutting each end of the "B" board. Also, always cut both ends of the boards the same. When the first end is complete, rotate the board end for end so the right edge of the board remains on the right (rotate away from you, not to the side) because the pin sequence needs to begin from the same board edge on both ends of each board. Pin first on the "A" board ends, and space first on the same edge of the "B" board ends.
Consider yourself a master if you get this all right on your first box jointed box and all 4 sides fit together the first time that you do this. Once you get this sequence right, it will be easy from then on.
Central North Carolina