I don't know if this counts as a malapropism or just a mispronunciation, and it concerns a non-English phrase. But I hear it quite frequently (apparently from people with just enough knowledge of French pronunciation to end up with some funny mispronunciations. The phrase is "coup de grace," which should be pronounced "coo de grahss"-- meaning, of course, "blow of grace." The common mispronunciation is "coo de grah," -- which is the way a Frenchman would pronounce "coup de gras" (meaning a "blow of fat").

Genie