Related: my mind was blown when I realize how easy it is to fall down to Earth (well, Kerbin) if you're in high enough orbit. I've had cases where my orbital velocity was on the order of 200 m/s, which means that at apoapsis you can just fart, and have the satellite crash into the ground a day later.

(I've also had my fair share of high-orbit missions where in the middle of the launch I realized I'm aiming for a prograde orbit, where the contract specified retrograde. Instead of reloading game from launchpad, I just carried on, put my apoapsis at target altitude, and once reaching it, burned off couple hundred m/s of Δv to reverse the orbit's direction.)