If you think about it--it is all in how we talk, what we mean and how we say it can shift the meaning in the words we're saying. If I say, "There's your best friend" to someone referring to a person they absolutely hate, sarcasm and tone of voice change the meaning of the actual words.
Sarcasm is just a part of this though.
We also have fun things (or horrible things) called puns:
Because words are just signals to real things, their meanings can often be misconstrued due to some form of communication discrepancy. But, more times than not, we goof up in our own language with things like homophones.
hmmm...homophones and sarcasm :)