I often say the wrong words.
I blame this on two things - routine and being too rushed. For example, when I am in the checkout lane at a supermarket, the last words are often, "have a nice day" - to which I reply, "thanks, you too." However, sometimes cashiers trip me up by saying something completely random like "enjoy your (whatever)" - and then my knee-jerk reply of "thanks, you too" seems kind of automated and wrong. Then, I feel like an ass.
Other times I catch myself mid-sentence and miserably try to correct the error with half of the wrong sentence and half of the right one. The mishmash of pleasantries is sometimes so confusing to the ears that all I can do is laugh.
