OK same thing happened on my 2012 this year. Riding around on my Whip, starting slipping out and then landed kite, checked board - fin missing.
My first thought was "North sucked". Plus I had a chip on my shoulder because North didn't respond to contact forms through website on several occasions prior to this. But I tempered my frustration with the chance that maybe I didn't tighten a screw down.
I contacted dealer not with the idea that it was a warranty issue but rather "I need a set of quad fins". Dealer told me $125 and I thought "No way I'm spending that to replace one fin".
Went to EBAY and found a brand new North Quad set for half that.
So I get the North Quad set, this time 2013 fins (same but blue color), and I notice that the notches on all the fins are off. Lengths of fins are off, width is off. Bottom line fins are cheap and not uniform - perhaps stamped out in bulk from China with low quality.
In order to make the new fin work I had to rebuild the notch with epoxy (ding dough) to the right size. I then went back to my original 3 remaining yellow 2012 fins and tweaked them for a better fit. Haven't lost one yet.
I truly believe that the fins' notches are off and that is why I lost a fin unexpectedly. I could get mad at North but I chose to suck it up.
Flash forward I buy a Dice 10m and the bar has got similar cheap plastic parts on bar ends that don't do what they're supposed to. Well the one side does but the other bar end doesn't snap off like in the North video. Also the plastic cap the holds the knots hidden is great on one side but on the other side it needs to be pried off with a knife. Again not like the video says "Changes on the fly are quick and easy"...
I could choose to be mad at North but I have found a good North support guy, Dan At North here on the board, and some good dealers like Ocean Air in Avon, East of Maui in De, 321 Kiteboarding in Fl. They are who North is to me.
The frustrating things I've experienced are minor but the gear is awesome so I'm rolling with the North crew!