inability to untap lands that were tapped by mistake

I find it amazing that there is no way to untap your land should you make a mistake when tapping it before highlighting a spell. It is just counter intuitive to play a spell first and THEN tap your mana, and I do this all the time. This leads me to quitting the game and starting over many times, right in the middle of an interesting game. You can cancel a spell, but how about the simple function of being able to untap your land and get the mana back? Any solutions to this problem? I am used to botarena, where you can tap all the lands you want, but untap them as well.
So far this is my only small issue with the program.
So far this is my only small issue with the program.