My first trip to Europe many years ago as a college student visiting family, I made the mistake of going directly to bed after landing in the morning. Slept until 3 p.m. and never got back on track. For the next two weeks I was up all night and sleeping most of the day and very cranky.
Second worst experience was my second trip to Singapore. I landed on a Friday and thought I'd given myself enough time to acclimate before beginning 4 consecutive days of training presentations on Sunday. I did okay for the first two days, but it really hit me on Tuesday. I remember standing up in front of the room, swaying back and forth, and my participants laughing at me saying, "You're jet-lagged!"