Probably the dumbest, certainly the one that I was most embarrassed of at the time. I ran into a colleague heading out from ARN while I was heading back in, just needing to catch a small domestic hop back to my then home. I had a time budget in mind and so we chatted as we got a burger from the local chain (Max) and then a coffee. I casually made my way back to Terminal 4, where a quick check of the board led me to the unpleasant realization that I had mixed up the flight's take-off and boarding times!
After quick trip to the nearby SAS desk with my head hung in shame, I was rebooked on the next flight a couple of hours later. I'd blame jetlag, but I've done similarly silly things with trains and public transportation in the US more often than I'd care to count.
At least I made it home the same day that time.