This technically isn't a regret but I am sad that I never got almost any Generation 1 BIONICLE sets I actually wanted.
See, I didn't get into BIONICLE until 2007 when I was eight and I took apart and lost the pieces to every BIONICLE set I got previously because I was obviously too immature and ignorant to appreciate and and take care of them.
So practically the only sets I had intact were the post-2007 sets with the accursed brittle ball sockets. You can probably guess how that turned out.
again, this really isn't a regret but a kind of sad story. Back in December 2008, I was in love with the set Maxilos and Spinax and desperately wanted it. I compiled an extensive wish list of system sets along with old BIONICLE sets I wanted like the Piraka and M&S. I still believed in Santa Claus at the time, you understand. I even specified on my list that Maxilos and Spinax was the one Item I wanted the most and that if I could only have one thing it would be that set. So on Christmas eve I went to bed with my mind full of eager expectation of the offerings I would discover under the tree in the morning.
I awoke the next morning to find the Indiana Jones set 7623 Temple Escape and the Agents set 8630 Gold Hunt under the tree, but not one BIONICLE set was to be found.
So not to sound too indignant but yeah, my BIONICLE G1 experience was pretty crappy and I suppose I am slightly bitter because of that. I also struggled to get invested in the complex story and never tried or had the money to buy titan sets or vehicles and I didn't really have money to buy many sets at all either. There is my sob story.