As a newbie, this is what I have experienced already.
1. Making tree huggers too short.
2. Fearing the sewing machine. (It's not that hard and it's actually fun making your own stuff.)
3. Whip and unwhip until you get it right.
4. Hanging the hammock too tight and too high.
5. Fearing knots. (Practice, practice, practice.)
6. Buying cheap, heavy, noisy, $10 tarp. (Invest in a good tarp.)
Still learning so I'm sure I've got a bunch more mistakes to stumble through.