This morning was interesting. I hiked maybe an hour through these mountains in my backyard.. not too far a hike, but long enough that when i finally got the phantom out of my backpack and set it up, calibrated, sat lock, look down at my app "NO SIM CARD" Dooh!!!! Then I remembered, my spare card I use to keep in my backpack is now in my Flytrex chip! And since it's the newest (well not anymore) Core 2, it still logs all the data on the chip until you install the card later and then it dumps the data to the card. So xfered the card to the camera... beautiful.. no show stoppers here... Then crap, left two of my props on the counter, luckily I have the spare props in another pocket! Sweet, another show stopper averted.. I do a test flight and bring her home, change the battery, and then I power up and get the 'Compass error' message via the app. The compass failure occurred when I had the phantom sitting on a huge rock rich in iron... excerpt from wikipedia... http://en.wikipedia.org/wiki/Magnetic_deviation "magnetic deviation often refers specifically to compass error caused by magnetized iron within a ship or aircraft... This iron has a mixture of permanent magnetization and an induced (temporary) magnetization that is induced by the Earth's magnetic field." I'm thinking **** I'm glad this app lets me know this before I fly. No problem, I do the compass dance, but it's not calibrating... red blinking lights... did it 4 times with no luck... Figured i would try one more time, finally, no blinking red lights, good satellite lock. I remembered Pullup's comment in another thread, perform a hover test before flying off... i did, and it did a major toilet bowl effect that I haven't seen in a while... i let it hover for a bit, and it eventually stabilized... but that told me enough info i needed.. don't take any chances, bring her back, hook her up to the assistant software to be safe and perform an advanced cali if needed. So who knows, maybe my experience and the help from this forum saved me from a pending accident. We'll never know.. but I have no regrets on playing it safe.. even with 4 full good batteries left..