I've just remembered a couple of excruciatingly embarassing malapropisms committed as a callow youth that I still blush at remembering:

- called to discuss some writer or other in class (Baudelaire maybe) earnestly describing him as a frotteur (I meant flaneur)

and

- talking to the youth club leader's beautiful young wife (on whom I had a huge crush) about the forthcoming production of the play she was rehearsing, and wondering if there was going to be a perineum arch (I meant a proscenium arch)

*shudders in horror at the memory*