Sustainable development strategies, including biomass waste valorization and life cycle-based carbon management approaches, are being adopted worldwide to mitigate climate change and reduce pollution. Numerous materials or products are under development to meet the demands related to a lowered carbon footprint, higher safety, access to renewable energy, and so on. One such material is biochar, which has been employed in a plethora of applications, areas ranging from soil amendment, polymer composites, energy storage and conversion, building materials, to environmental remediation. However, no two biochars are created equal. This disparity in inherent material properties in biochar means that they should be subjected to effective engineering to tailor their properties to a specific targeted application. This means that a biochar that is suitable for concrete applications may not be appropriate for polymer composite applications. Even biochar properties that were designed for a specific application may not have the desired or expected impact. This can lead to several negative effects of biochar. Hence, this article has collated and reviewed the instances where biochar has had unintended consequences in different applications. Using the knowledge presented herein, researchers can be aware of the potential disadvantages of biochar applications and take necessary steps to abate the negative effects.