I was going to mention spoonerisms, but they're not really that funny. However, thinking of Dr Spooner put me in mind of an anecdote about another man of letters, Dr Johnson. He was pretty fastidious about the correct use of lanugage, but not too hot on the personal hygiene front. When his landlady said to him one day "Dr Johnson, you smell", the great man replied "No madam, you smell, I stink!!"