Cold Feet: Open Source SDK and realize, less than ten seconds later, that you're not interested in mapping at the moment.
Give Up Yet?: Survive a four hour long compile of your first map, before learning about func_detail.
Mini-Me: Create a well thought out, well planned Dev map, and then load it into TF2 to realize you have the scale completely off.
No Need to Defend: Create a team_control_point, together with the prop, but forget to make a trigger_capture_area.
Snake, You Can't Do That!: Rename your .bsp file after compiling, building cubemaps, and paking all custom material.
Snake, You Can't Do That!, Part 2: Release the renamed .bsp publicly, and become hopelessly confused when people report missing custom material.