A researcher conducts a study to test a new pill advertised to improve memory. They recruit 50 participants and divide them into two groups. Group A is explicitly told they are receiving the new 'memory-enhancing' pill. Group B is told they are receiving a placebo, a sugar pill with no active ingredients. After one hour, both groups are given the same memory test. The results show that Group A scored significantly higher than Group B. Based on this design, what is the most significant flaw that challenges the conclusion that the pill itself caused the improved memory scores? Explain your reasoning.