It can be very funny when words get mixed up inadvertantly because they may sound like what you meant to say/or you have some kind of word association with it. Often times this gives what you are saying a completely different meaning to what you intended.
My brother in law to be was sick recently and I was chatting to my parents about it and as my mind searched for the word desribing what was wrong, I came out with "rickets" when I meant to say "shingles", much to the hilarity of my parents.
A great example I heard on the radio yesterday was a guy who was going through a run of bad luck said something like: "what's next, famine and petchulance?" Think you meant pestilance there hon!
Or on the Apprentice a couple of weeks ago, Jenny complained that she ended up having to "breast feed" two members of her team, meaning spoon feed!
Can you think of any other examples?