I am lying to you.
Analyze this proposition for truth value. Here is a great summary of the problem:
"if true, it would imply that the speaker is lying about lying and therefore telling the truth, rendering the statement false; if false, it means that he or she is in fact telling the truth, which would imply that he or she is lying to us, rendering the statement true. This is a paradox that seems to trap us every way we look at it."
http://www.galilean-library.org/int10.html