Today being Ash Wednesday, I decided to omit having breakfast and to abstain from meat by having paelle thinking it was a fish-only meal. But, on chewing, I tasted chicken and read the packet and confirmed that it was chicken. Have I broken the rules and commited a serious sin?
No. Assuming that you stopped eating the paelle once you realized your mistake, all you did was make a mistake. Sin requires knowledge and consent. Innocent mistakes are not sinful.