And the worst experience developing for GPS mobile applications goes to....

... not carrying the development environment with you when real testing the application. Ok, I could carry my laptop around to our drivings, but I'm on holidays, it is hard enough to explain to my wife and daughter using the laptop on the hotel...

Yeap, it's darn disappointing to find you left an annoying balloon looping over you application when some "it will never happen" scenario you did anticipate but didn't bother to treat comes together... Let's all (most of us to be more precise) tank God for not having to write critical software for an intergalactic probe - we'd probably find it too stressful not to be able to correct one's mistakes...