They were not able to be discharged until they agreed with the psychiatrist's diagnosis and received antipsychotic medication. (They flushed it down the toilet.) While it's not stated in the above, at least I don't think, I know that a certain (small) number of hallucinations are actually quite typical, and you don't realize it because they look like ordinary objects, for example. It was from a cracked.com article... I don't remember which one.
The other part of the experiment was telling the staff of an institution that they were going to send them pseudopatients and asked them to identify who they may be. They didn't actually send anyone, and so the staff misidentified ordinary patients as being impostors.
Doesn't give you much faith in the reliability of the whole system. The pseudopatients themselves had thought they could be discharged much earlier than they were, simply because they were actually not mentally ill. Definitely worth a read.