I've done some projects with IDE 1.0.6 in the past and after a period of doing nothing with Aruino I have started a new project (MPPT buck/boost charge controller for a small wind turbine). After upgrading to IDE 1.6.6 I found out that most of my projects don't compile or run flawlessly anymore. Even running a standard program like 'Hello World' with a LiquidCrystal_I2C library doesn't run as before after compiling with IDE 1.6.6; only the letter 'H' is visible in the display. After hours and hours of trying to find out what is wrong, I decided to revert back to 1.0.6 and much to my surprise everything is compiling and running smooth again. Backwards compatibility seems not to have been one of the main requirements of version 1.6.x but of course I hope that I'm wrong... :P It would be great if anyone of the community can give me some tips on how to avoid these issues in 1.6.x.