I built a lander,and to test out docking and decoupling abilities I used gravity hack.Lander was working good.Then I built a launcher for that lander.The whole rocket was looking very good,but I wasn't sure can it fly.Then I launched it.It flied flawlessly! I said "Oh my god! Did I really build a BIG,COOL LOOKING rocket that FLIES!?!".But when there was time for gravity turn,I switched to map view,and saw I'm on escape trajectory out of kerbin. I forgot to turn hacked gravity off. anyway,here's a happy ending.I modified my rocket a little bit,and it was capable to fly again!