11 "'You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes his name in vain. (ESV)
This one seems simple, but lets really think about what vain means. Vanity implies a connotation of being artificially puffed up. I think this is more than a ban on saying "Oh my God!" During the time it was written, it was common for people to invoke the name of their god "We will win. by Baul." This is warning not to tread lightly with using God's name to add authority to causes He has not called His people to go after. It is easy to reach for the name of God to justify actions, to urge support for a cause and to puff ones self up.
This commandment also holds an extra warning, like the last one did; he will not hold you guiltless for violating this commandment. It seems like a small infraction to us, but it is a sin worthy of the ten commandments, and must be repented of like any other. He is still God, and His ways are still higher than our ways; Hist standards are still higher than our standards.