I would call myself an experienced sewer, I have been doing it a long time and have done a variety of projects. However, it seems that there are some lessons that I am destined to repeat. Chalk it up to stubborness, everlasting optimism or just plain stupidity (I fear it is the latter) but here is my list:
1. Pattern sizing is wonky (it's not my body, I swear it is the pattern). Make a mockup before you cut into that expensive/favorite fabric to get the proper fit.
2. Pattern directions don't always give you the most professional look. Patterns are geared toward the lowest common denomenator, meaning a beginner. Read through them and adjust your methods accordingly to get the best results.
3. Take time to look where you are cutting before you actually cut. Pick up the whole garment and look because once you cut that perfectly done french seam that was lying under that seam you were trying to trim, there is no fixing it (well you can fix it...)
4. Irons are hot. Irons produce steam which is hot. Pressing the steam button when your hand is an inch away from the iron will burn.
5. It IS possible to sew fingers on the sewing machine. A sewing needle can penetrate a nail and come out the other side of your finger. And when you are looking stupidly at your finger (or thumb as the case may be) which you just harpooned, and carefully extract it from its position, thankful that the stupid needle (yes, now it is the needle's fault) didn't break off IN your finger, remember that it is going to hurt. A lot. Keep Neosporin and band-aids handy.
And the most important lesson of all that eludes me:
6. Being in a hurry is mutally exclusive from doing a job well. Most of the above mistakes come from being in a hurry. You have a picture in your head of you wearing the finished project, it is going to be gorgeous and everyone asks you about it. They are awed by your skills and creativity. But to get from that picture in your head to the fabulous finished project, you need to slow down. Take your time and do it right.