Multi-subject legislation presents challenges that can be examined from a dual perspective. On the one hand, it may involve procedural flaws in the legislative process, if the democratic principle has been violated; on the other hand, it reflects poor legislative drafting techniques and jeopardize the principle of legal certainty. To analyze this phenomenon, the various categories it encompasses will be studied and some practical examples will be reviewed. Additionally, there will be an in-depth exploration of the applicable regulations, particularly those related to the core provisions. Lastly, the aforementioned drawbacks and their potential impact on the validity of legal norms will be further examined.