Let's say my friend has a fly on her head, but she does not know it.
I say to her: "You don't know that you have a fly on your head".
Right before I uttered my statement - it was true, but right after I uttered it - it was false.
Was I lying?
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The timing of the statement is not paradoxical at all. If you state truthfully that someone doesn't know something, then you haven't lied in uttering that statement. The fact that the target of your statement will then know what they didn't previously cannot undo the truth that was uttered in the past. If you repeat the statement however, then you would be uttering a false statement due to the other person subsequently being in possession of that knowledge.
Let's mix this up a bit. You are about to tell your friend that she has a fly on her head, and you knew a moment ago that she was not aware of this, but as you are about to utter your statement, your friend sees the fly in a reflection. You then say "you don't know there is a fly on your head". Have you lied?
Clearly not. Your knowledge may be in error and the statement false, yet this does not make your statement a lie. This I think is the crux of Michael Dorfman's answer.