My first 4 stroke bike, KLR 250, was hard to start when I bought it. I had to push it all the time. got worse and worse till no vroom vroom. Started tinkering and found the valves were not closing. I discovered the tapet adjusters and just turned em out untill the valves would hold gas in the intake and exhaust ports. Feeler guages? Sure! if she lets me I will! I put it all back together and felt like a hero when the bike started. Ran the bike out in the field with a big smile, then, the most gut wrecnching mechanical sound I ever herd as the nut on the tapet I forgot to adjust got into the timing chain and caused it to break shattering the valve cover and bending the crank among other things ..... No more KLR :( .
I spent 4.5 hours installing a new set of front leaf springs in a truck wich was a good time for the job. Some one esle had taken the springs out to get the measurements. I put the new ones in and go to drive the truck out, turn left out of the bay, then right,,,, the wheel dose not turn right more than 1/4 way! Back the truck back in and start looking it over, takes me aoubt 10 mins to relize they had an offset center pin and I put them in backwards! The 4.5 hour job went to 6 hours. Dohhhhh!
Oh another good one, Had a flat on my bike as a youth, so I fixed it and rode it to school the next day. On the way home to show off to all the kids I yank a big ole wheelie on the way out the lot, how surprised I am to see my front wheel keep going, forks come down, hit the pavment, I go over the bars in a heap. It hurt, but I was so embarassed. I looked up and OMG! No one had seen it! I hobbled over to the bike and collected the front wheel and hobbled around the corner to agonise in pain :p .
Other than that, just dumb stuff like putting your front wheel on with the disc on the wrong side :)