I regret marrying someone who already had children. It's been eleven years of unhappiness, dealing with the kids (now 22 and 19) and the ex wife.
I regret not bringing our daughter home after she passed away. We were asked by the funeral director if we wanted to bring her home for a while and we said, "no" because I was afraid that if we did, I wouldn't be able to let her go again. Every day I regret not putting her in her bed for one last time.