I regret my attempt at classical education. I'm a language and literature bent kind of gal, but my children are maths and science bent (like my husband). My eldest HATED classical education, and thrives much more with a traditional scholastic approach, heavy on the maths and sciences; my son prefers the same. If I'm honest, we ALL prefer a more school-at-home traditional scholastic approach - it's where I teach best. Looking back, I have some serious qualms with the classical approach, including but not limited to that most of the resources, as written, are ridiculously science and math "lite".
ive only been at this since Sept with one kid, but I regret buying a few pieces of curriculum. I got them becuse they were very "public school" and I didn't know if I'd get any type of hassle from the district on approving my home ed plan, so I purposely got them to appease bureaucrat people that I didn't even know, nor had any idea what they were thinking or if they even cared!! SO, next year, I"m buying fewer curriculum books like your standards school-type workbooks and textbooks and am going to focus instead on real children's literature books and student-created projects to go along with them.
I can't say this part is a true regret: but I do spend a lot of time wondering if I could have/should have tried to homeschool all my kids from K throught grade 5... I think they'd be better readers and writers at this point if I had. But, I surely did not know then what I know now, so it could have been a dismal failure too.
Honestly, I'm not sure that I have one yet. I went to my first expo when DD was 18 mo, knowing she needed to be HS'd. I like our programs and so far it's going really well. Sure there are days where I could plan a little more or focus a bit more but I also know that we're so far ahead that it just doesn't matter in the big scheme of her education thus far.
I do wish that I knew another language. DH speaks only French (he is also fluent in Spanish) to our kids and I wish I could offer another to give them that advantage but we will just deal with that in the future.
When I look back over our homeschooling career, everything that I regret has the positive note of leading us to where we are. If I had just done it from the beginning, I may have gotten discouraged and put them back in which would have been even more of a set back. If we hadn't tried K12, I wouldn't have gotten my feet under me and learned to let go. If I hadn't tried the curriculums that weren't a good fit, I wouldn't have tried designing my own for some of them. And doing science and language arts on our own has been such an amazing success! So yeah, I regret staying with K12 a long time, I regret using the Delights book for language arts, but I wouldn't be on this path right now.