-Used a crank arm puller and forgot to remove the bolt that held the arm.
-Mounting tires with the rotation arrow backwards.
-Cutting a threadless steerer tube a wee bit too short.
-Mounting an inner tube with a double twist in it, so it's like a balloon dog inside the tire.Hey it was OK til I had a punctture a year or too later,
-Cutting a little metal off a seatpost tube to allow a lower seat and finding that the remaining stub was too thick for a quick release clamp. Had to use a boot on clamp.