I have made lots of mistakes - but I think the biggest one that I could have avoided if I had listened to my gut and my child was
letting a curriculum vendor convince me that my very bright child could go ahead into Pre-Alg/Alg with his program. He could do the work - mechanically, but he just wasn't ready to go there. So we ended up spending more time and effort going back and picking up things he didn't really get. He is an extremely bright kid - but he thinks he is bad at math. :(