This research paper investigates the phenomenon of hallucinations in large language models (LLMs), focusing on distinguishing between two types: hallucinations caused by a lack of knowledge (HK-) and hallucinations that occur despite the LLM having the necessary knowledge (HK+). The authors introduce a novel methodology called WACK (Wrong Answers despite having Correct Knowledge), which constructs model-specific datasets to identify these different types of hallucinations. The paper demonstrates that LLMs’ internal states can be used to distinguish between these two types of hallucinations, and that model-specific datasets are more effective for detecting HK+ hallucinations compared to generic datasets. The study highlights the importance of understanding and mitigating these different types of hallucinations to improve the reliability and accuracy of LLMs.