a doctor told a story once (in an article in NYT) about being careful what you say as patients come out of anesthetic, since they often hear things before they are full conscious.
His patient had a "galloping heart" (rate) which the doctor mentioned-- the other doctors all nodded, and were concerned-- after open heart surgery "galloping heart" is not a good thing.
But a few days latter, the patient was fully recovered, and on his way home. He commented to the doctor that it was something he heard the doctor say as he was regaining consciousness that made all the difference. the patient knew going into surgery that he was high risk, and that there could be serious complications, but when he heard the doctor talk about his galloping heart--He knew he was a strong as horse, and was going to make it!
not quite a malapropism-- but a sort of "mis hearing"
