The dumbest thing I have done lately is debate politics with my father-in-law. When the man doesn't like you very much to begin with, it is never a wise decision to actually tell him what you think. After an hour of his ranting and criticism, I have since learned that it is always a good strategy to nod your head and say "That's interesting", then change the subject. After being married for a few years, I have learned that this is also a good strategy for marital harmony, and has saved me from countless other instances of the dumbest things I could have done.
In high school, I dated yet another girl whose father couldn't stand me (my luck, I suppose). While at their home one Saturday, I decided I would help out with the chores around the house. Knowing that the father took great pride in his lawn and its appearance, and knowing that even I (the bumbling, stupid, adolescent boyfriend) could handle something so simple as mowing the law, I hopped on the riding lawn mower, revved it up, and drove down the driveway to cut the grass in the front section of the lawn. My girlfriend yelled out that the lever on the left lowered the blades, and to be sure to get around the mailbox really well. As it turns out, I (the bumbling, stupid, adolescent boyfriend) could NOT handle something so simple as mowing the lawn. After 20 yards or so, I looked behind me and saw nothing but clumped, mutilated sod and dirt where a beautiful stretch of lawn had once been. The lever on the left, I found out, lowered the blades to different levels, and I had apparently chosen the "Total Grass Annihilation" level. I turned the mower off and promptly left. Needless to say, my relationship with her father was somewhat more strained after that.