It's unclear if this experiment actually happened the way Rosenhan claimed. A journalist went through Rosenhan's archives and tried to verify his story. She managed to track down one of the pseudopatients, who disputed some of Rosenhan's claims such as the amount of preparation, and whether Rosenhan had worked out a legal backup plan in case the institution refused to release the patient.[1] She also noted large discrepancies in various numbers. Apparently she wrote a book about the whole thing, but I haven't had the chance to read it.[2][3]

1. https://sci-hub.red/10.1038/d41586-019-03268-y

2. https://www.npr.org/2019/11/13/777172316/the-great-pretender...

3. https://www.susannahcahalan.com/the-great-pretender