My noob mistakes from earliest to latest.
1st-Overwatering seedlings and in early veg which attracted gnats and caused other issues.
Only water when pots are almost completely dry, this is especially true in veg.
2nd-Feeding nutes too early at too high of strength. Getting to experience your first nute burn, not priceless. I'm not sure if anyone can say they've never experienced it.
It takes some experience to know the best time to begin nutes but it's easy to just start out at 1/4 the recommended strength and work your way safely up to each plants specific needs.
3rd-Began flushing too early. Early on I went by the flowering times issued by breeders, and I began counting days from 12/12 instead of first signs of pistils like one should. I got almost full cloudy trichs and began flushing. I went 32 days of water only before I got my 10-20% I like. About 3 weeks over estimated breeder times. It was some of the cleanest smoke I ever had but I definitely lost yield and possible potency.
Breeder times are good to use to estimate but you must begin counting days at or soon after signs of first white pistils rather then the 12/12 switch. And always prepare to go at least 1 week over breeder times. You must also factor in any issues you had IMO and IMExp. If you burnt your plants in flower, or whatever issue in flower, and you spent a week or more recovering, you just lost that week. I don't know how true this is outdoors, but indoors any time I've had issues in flower my flowering times were longer then normal. I suppose the same could be said if one didn't have enough lumens, or watts if you prefer, to properly flower your girls. I remember when I was doing half the lumens in the room I have now and flowering times then went on much longer then they do now.
4th-Messing around with lighting schedules and tricks.
It's simple. 18/6 veg-------12/12 flower. Nothing else works better, why mess around?